Unit Confusion

""Light-years isn't time! It measures distance!""

- Boy in Brock's Gym, Pokémon Red and Blue

Many times when a measurement is given, the units of measurement don't make sense for what is being measured. This is especially annoying when the character suffering from Unit Confusion is supposed to be a scientific genius.

This occurs frequently in Science Fiction. Compare Sci-Fi Writers Have No Sense of Scale. Averted when the writers use Fantastic Measurement System, as the writers can redefine time units whenever they like. If they care to keep it in mind.

General

 * A frequent instance of Unit Confusion is when "watts" is used with a unit of time, as in "watts per second". "Watts" already incorporates a rate (one watt = one joule per second) so "watts per second" only makes sense if referring to a power source that is changing in intensity. A similar mistake is the use of "watts" when it is really a quantity of energy that is meant, as in "this battery can store up to 200 megawatts of power", when the accurate thing would be to measure the stored energy in joules (watt-seconds), watt-hours (1 Wh = 3600 J), or as technicians usually do, in ampere-hours (which when multiplied by the battery's voltage give watt-hours).
 * In all fairness, this is a common mistake in general. Velocity, for example, is technically a speed and a direction, but a direction isn't always given, and "speed" just doesn't sound scientific enough. Also see Department of Redundancy Department.
 * A mistake that seems to be getting less common (but it still showed up in, eg., Earth 2): Like parsecs, light-years are a unit of distance (the distance light travels in a year), not time. Whether or not the phrase "light-years more advanced" is a mistake or just an analogy (as in "miles ahead") is debatable, and probably varies from case to case.
 * People mistook "light-years" to be a construction along the lines of "Space Miles" or "Earth Minutes" - sciencey word + unit of measure = sci-fi unit of measure.
 * To increase the confusion a bit more, the second in parsec (parallax second) is a unit of angle (an arcsecond), while the year in light-year is indeed a unit of time.
 * One obscure enough that it comes up all the time: Since 1968, the standard unit of absolute temperature is "kelvins", not "degrees Kelvin". The older usage dates to before the 13th General Conference on Weights and Measures (CGPM).
 * On a related note, some works make reference to a temperature which is lower than absolute zero. Absolute zero occurs in the total absence of heat or energy of any kind. You can't get colder than that.
 * In this universe, anyway. In Discworld, you can get so not-drunk you go knurd, and the fifth horseman of Apocalypse has a sword so cold it creates coldness beyond absolute zero.
 * It is, strictly speaking, possible for a temperature to be recorded as a negative number of kelvins - somewhat ironically, a temperature expressed as a negative number of kelvins is hotter than any positive number. That said, it's difficult to imagine it being used correctly without sounding like a Critical Research Failure
 * Another related fallacy is the assumption that, for example, 40 °C is twice as hot as 20 °C. This is not true because 20 °C is equal to 293.15K; twice as hot as this would be a lot more than 313.15 °C (though not as much more as you might think; the relationship between heat and temperature is exponential rather than linear).
 * Perhaps not, strictly speaking, a unit, but "AD", unlike "BC" and "CE", is supposed to come before the year, so "2005 AD" is incorrect. This particular error is so near-universal both in and out of television that it's even accepted (and used) by professional historians.
 * Another frequent confusion with casual users of A.D. comes from the (incorrect) notion that it stands for "After Death." Their thinking is that years B.C. were years before Christ was born, and years A.D. were years after Christ died, with 30-some-odd years passing in between that weren't numbered. In reality, A.D. stands for Anno Domini ("in the year of our Lord") and was supposed to mark the year that Christ was born, so the day after 31-December-1 B.C. was 1-January-A.D. 1. Note that there is no year 0, and due to old calculation errors, historians now believe that Jesus was born in the single digits B.C.
 * Some astronomers use a year zero to facilitate calculations between years (namely, subtraction). This is also why you'll see years given as negative numbers. It's much easier to do math by hand or by programming if you don't have to make strange exceptions for how numbers work at any given point.
 * Other astronomers use a Julian Day Number to describe dates, and that has no "year" component at all.
 * Confusion between metric units and American Customary Measurements occurs all too often, in TV and in real life, where a person accustomed to using one may not realize that a foreigner is giving them numbers in meters rather than feet.
 * In TV, Hilarity Ensues. An example from Last of the Summer Wine involves Wesley assuming the pretentious Seymour used metres rather than feet in his design for a kite, with the result being the size of a hang glider.
 * In real life, it causes Mars Climate Orbiters to disappear, or planes to run out of fuel half-way through their scheduled trip.
 * Confusion between Imperial and American Customary measurements is common as well. The two share the names of measurements, and lengths are the same value in both, but not volumes - pints and gallons for example.
 * In relativistic calculations, the speed of light is often set to the unitless number 1, as this simplifies equations (for example, E=mc2 becomes E=m). This makes time and space use the same units, so that a light-year actually does measure time as well as space (it's equivalent to one year). It's doubtful that this justifies any of the other examples on this page.
 * One of the major, earth-shattering revelations of relativity was that spatial and temporal dimensions are, in some sense, the "same thing". One can, in fact, use units of time and distance interchangeably, and be correct—i.e. you can talk about "years of distance" or "kilometers of time" and be technically correct. If you work with relativity a lot, you'll get pretty used to using just "year" as a unit of distance—which is perfectly correct.
 * You could also use distances to measure time—which isn't commonly done, because a kilometer of time, for example, is a very small amount of time—around about 1/300,000 of a second. But "kilometers of time" are seen in long-distance telecommunications, where signals take noticeable time to propagate, and even "millimeters of time" become important in designing circuits that switch billions of times per second, such as those in a modern CPU.
 * With some mucking around with unit conversions it is possible to measure mass in units of distance. Starting from kilograms, multiply by G (the gravitational constant) then divide by c2. This leaves you with your mass in meters, which will be very small (the Earth's mass is about 9 millimeters).
 * Incidentally, this is the formula for a Schwarzchild radius (i.e. where a black hole's event horizon is). So a black hole with a mass equal to Earth's would be 9 mm in radius, or 18 mm across. As a quick approximation, a black hole's radius is 3 km for every Solar mass it has. A theoretical black hole with a mass equal to the mass of the observable universe would have a radius about equal to the radius of the observable universe.
 * The confusion regarding storage space. To a computer, a kilobyte is 1024 bytes, a megabyte is 1024 kilobytes, a gigabyte is 1024 megabytes and so on. However, to hard drive manufacturers, a kilobyte is 1000 bytes, a megabyte is 1000 kilobytes, etc. To avoid confusion in recent times, the binary prefix system was invented. So a kibibyte is 1024 bytes and a kilobyte is 1000 bytes. Still, the old way is interchangeable.
 * Mac OS X Snow Leopard and recent versions of Ubuntu report space with each order of magnitude increasing every 1000.
 * The old High Density 3.5-inch floppy disks were marketed as being "1.44 MB". Their actual storage capacity (before formatting) was neither 1.44 x 1000 x 1000 bytes, nor 1.44 x 1024 x 1024. It was 1.44 x 1000 x 1024 bytes, or 1440 Kilobytes.
 * Hard drive manufacturers most often use the convention of one billion bytes to the gigabyte to make their products' capacities sound bigger. Even flash memory chips, which are manufactured in powers of 2, report their size in powers of 10 so that they can use the extra 5% to 7% as spares in case sectors of the chip wear out.
 * "Kilo" and "Mega" are exactly 1000 and one million, respectively, when describing data transfer rates in bits per second; e.g. a 56 KBPS analog modem transfers data at 56,000 bits per second, not 57,344 (56 x 1024) bits per second. However, the number of kilobytes transferred per second is in increments of 1024 bytes, to match the numbers reported by the file system.

Anime and Manga

 * The first English dub of Dragonball Z falls victim to this trope: When Bulma is explaining to the group why traveling to Namek would be impossible, she gives her answer in years, then promptly adds "And that's in light-years!" Perhaps Bulma is distinguishing years in an ordinary slower-than-light craft from years at c.
 * In Bleach, Ryuken tells his injured son that if he yell 5 Hertz louder he'll reopen his wounds (basically telling him to shut up), but Hertz is a measure of frequency (high or low) not intensity (decibels in the case of sound). But then Ishida is kind of a Glass Cannon, so to speak, and the area around a wound may have a resonant frequency that he almost hit.

Comics

 * Parodied/subverted in a FoxTrot strip: Jason is playing a racing videogame while his mother is trying to get him to come to dinner. Jason, who had earlier said "Just a sec," clarifies that he meant a parsec; just until he drove an entire parsec in the game, which would take him over 10 million years.
 * Another time, Jason decided to take up baking, and mused over whether the 350 degrees he had to set the oven at were in Celsius, Fahrenheit, or kelvins. Peter sarcastically suggested that he rotate the oven almost a full circle. "Don't be silly, Peter."
 * Parodied in Frazz, when Caulfield points out rather loudly that light-years are a measure of distance. The teacher then tells him to quiet down because he's "making a ton of noise."
 * A Mafalda strip had a teacher giving her students a math word problem where they had to calculate the area of a field measuring X by Y hectares. Amusingly enough, a later compilation featured an entire page of sketches by the author mocking himself for the mistake (the best is one where Susanita says "Hey Quino, you big idiot! How many liters are there in a kilometer?")
 * This Invincible Iron Man panel gets the unit type right but royally screws up the SI prefix: a "picobyte" would be one trillionth of a byte. Must be really optimized code. Osborn probably meant to say "petabyte" or 2^50 bytes.
 * In the Dutch comic "Heinz", the eponymous character asks what a light-year is, and the specialist responds that it is a year in which everything goes off without a hitch. This might be more of a malapropism, though.
 * Scrooge McDuck has often boasted of his money bin storing "three cubic acres" of cash. An acre, of course, is a two-dimensional shape, and cubing it would create a geometrically-impossible six-dimensional shape. Although that would go a long way towards explaining just how all those countless billions fit inside that single building.

Films

 * The most famous example of all: Han Solo bragging that his ship can make the Kessel Run in "less than twelve parsecs". Parsecs are a unit of distance, not time. While this has been Retcon-ned to death with constantly shifting backstories about skilled pilots shaving distance by flying close to black holes, even those omit that the Parallax Second is based on local (terrestrial) factors, specifically the radius of the Earth's orbit, and would have no meaning in "a galaxy far, far away" though the term could refer to a planet with a similar orbit, like Coruscant (could be a translation convention, Han sounds like he speaks English after all).
 * Though even George Lucas seems to have forgotten this (see next bullet point), this error seems to have been intentional and meant to paint Han as a blowhard. Not only does the script pretty much say so (and earlier drafts moreso), but if you watch the scene closely, Ben kind of rolls his eyes, seeing through Han's BS.
 * George himself gave an (awkward) explanation in the DVD Commentary. The Kessel Run is through a very nasty piece of space filled with all sorts of hazards. Lesser ships plot a longer but safer route. Because speed is a function of distance and time, you can decrease the time and the distance that it takes to travel between points by using a more powerful engine and more efficient computer that can plot a more direct route through hyperspace. Of course, this means Han's "more direct route" took him through over forty light-years.
 * Another explanation offered was that the Kessel Run was between multiple points that were moving apart from each other, starting from Kessel. The implication was that to visit each point with a total traveled distance of only twelve parsecs was quite fast indeed.
 * The Kessel Run competition could just as well be, you've got X minutes/hours/days, let's see how far you'll get.
 * It could just be that "parsec" means something completely different in the Star Wars universe and they are just homonyms.
 * In a rather literal meta-example of this trope, there was much confusion over the definition of the term "unit" from Attack of the Clones (referring to the two hundred thousand units being ready and a million more well on the way). Many assumed it referred to a military unit, and took issue when it turned out to refer to an individual clone. The thing is, it had always referred to an individual clone, even before the movie came out as the Attack of the Clones movie novelization (which came out about a month before the movie did) pretty much told us that "unit" referred to a unit of production, and matched the "million more well on the way" statement with "a million clone warriors".
 * In Plan 9 from Outer Space, one of the aliens says "Foolish humans, we have had space travel for eons of your light-years." An aeon being one billion years and a light-year being a measure of distance...
 * Used intentionally as a major plot point in This Is Spinal Tap. While trying to work out a big, stadium-pleasing onstage stunt to help the show, Nigel draws plans for a Stonehenge monument on a diner napkin, and the monument is built to the exact specifications he wrote, 18 inches (18"), not 18 feet (18'). This leads to "a Stonehenge monument on the stage that was in danger of being crushed by a dwarf."
 * "These go to 11."
 * In the movie Stealth, the processing speed of the AI was given in "terabits per second". This is not a unit of processing speed, but throughput. Processing speed would probably be measured in MIPS (millions of instructions per second) or flops (floating point operations per second), though the processing speed needed to keep up with I/O rates in the Tbps would still have to be impressive.

Literature

 * Often times Jules Vernes' books contained measurements and spot on calculations. However, when they were brought over to the UK and US the translator screwed up horribly on the conversions at first. Partly by not really even doing them. Just substituting the metric unit with the rough alternative, but keeping the numbers.
 * Many people like to sound clever by pointing out that Twenty Thousand Leagues Under the Sea is 60,000 miles, and thus impossibly deep (the deepest part of the ocean is less than 3 leagues down), but this figure is the distance travelled in the book, not the depth they reach.
 * Used in Robert Asprin's Myth Adventures series; as a dimension-traveler stranded in a backwards world, Aahz has trouble with Klahdish units of time and often has to be corrected by his apprentice Skeeve.
 * In Encyclopedia Brown's defense, one story referenced this trope in its solution: a man claiming to be a sailor is revealed as a fraud because (among other mistakes) he refers to speed in terms of "knots per hour", when a knot is already a measure of time over distance, i.e. one nautical mile per hour.
 * It should be noted that a "knot" can also refer to the distance between knots on a log line, and used to be an acceptable term for the unit of speed.
 * In modern usage, a "knot" is "one nautical mile per hour". The Other Wiki has the details of how it all fits together, including the fact that knots on a line were historically counted out over roughly half a minute, not an hour.
 * Perry Rhodan plays with, then in-setting justifies the "temperatures below absolute zero" version: in the first arc in which humanity and allies seriously and consciously have to fight off the self-styled forces of cosmic chaos, one of the latter's weapons is a phenomenon that does indeed cause objects caught in it to rapidly cool down to absolute zero and below...whereupon they simply vanish from a universe in which they can no longer exist. In reality, they end up getting displaced into another universe whose own 'absolute zero' is in fact several hundred kelvins below that of ours, a fact that is discovered once actual survivors of the whole process start to come back...
 * In a travel guide by comedian Dave Barry he facetiously lists a kilometer as a unit of foreign currency.
 * In The Screwtape Letters Screwtape uses "light-year" as a measurement of time.
 * Fahrenheit 451, supposedly titled after the flash point of paper. Which is, actually, about 451 degrees Celsius.
 * In The Truth about Pyecraft by H. G. Wells, A fat man called Pyecraft persuades the Narrator (who has some magical recipes from his great-grandmother) to give him a potion to let him lose weight. Unfortuantely, while Pyecraft does lose Weight, he doesn't lose Mass, and ends up floating near the ceiling.
 * Earth (The Book) ends with a survey for the reader to full out as an "Application for Genetic Reconstitution by Aliens." One of the questions is "How many kilowatts would you say your brain produces hourly?"

Live Action TV
"Clark: You're saying Kara's actually on Krypton. Chloe: Well, Krypton 1989. Give or take a few light-years."
 * Bones once referred to "Force in Newton-Meters." Unfortunately, the newton-meter is a unit of torque, equivalent to one joule per radian.
 * The Newton-meter is also a unit of energy equivalent to the joule (as radians are dimensionless); in this case, the meter is the linear distance over which the force is applied rather than the distance from the fulcrum. Nevertheless, it's still not a unit of force, which is simply Newtons.
 * Even Myth Busters made the "watts" mistake, displaying a chart showing the power consumption of light bulbs in "watts per hour".
 * On an episode of Are You Smarter than a Fifth Grader, one of the quiz questions was "How many watts are used during one kilowatt hour?" (The answer given was 1,000, but the question as written is unanswerable since one kilowatt-hour can result from using 2000 watts over a half-hour period, 500 watts over a two-hour period, or any of an infinity of other possible combinations.)
 * Not to mention the fact that the phrase "during one kilowatt hour" is just plain wrong as it implies a kilowatt hour is a unit of time (it is a unit of energy, just like joules).
 * In an episode of Star Trek: Voyager, the power output of the warp core is given in "teradynes per second". Dynes are a unit of force, not energy.
 * Although this could just mean that the warp core's power could be used to exert X teradynes of force.
 * Several Star Trek episodes have used the word "teradyne" as a unit of computer processing power. "Dyne" in the Trek universe seems to mean "a small amount of whatever it is we're talking about right now."
 * The Voyager writers had trouble even keeping their own fictional units straight. On at least one occasion, "cochranes" were used (quite incorrectly) as a measure of explosive yield, rather than the proper usage as a measure of warp field strength.
 * To be fair, real life nuclear explosive yield figures tend to be in kilotons and megatons (equivalent of TNT), instead of joules. Maybe people in Voyager still have no grasp of how much a joule is, so use some kind of Cochrane equivalence instead.
 * In one novel, they used something called a "Cochrane radius", which is basically the minimum distance from a star at which it is safe to engage a warp drive. If a warp field is created within that radius, the star would go nova. Of course, since this has happened on several occasions in the show without any ill effects, it can be safely said that this is bullshit.
 * The episode of Star Trek: The Next Generation "The Royale" listed a planet's surface as being -291 °C. Not only is it below absolute zero (-273.15 °C), but it's really, really unlikely to occur naturally on a planet, thanks to the physics explained in Space Is Cold. This was due to someone deciding to swap out the word "Fahrenheit" with "Celsius" at the last minute.
 * The light-year confusion is brought up in an episode of The Big Bang Theory. Penny is talking to Sheldon (a theoretical physicist) and says something like "I guess you would say light-years ahead of his time". Sheldon responds, "I wouldn't say that. Nobody would. Nobody who knows what they're talking about, anyway."
 * Chloe Sullivan also does this a couple of times in Smallville. For instance (in the context of a discussion involving time travel):

"Narrator: The entire crew of the Nautilus - all 20,000 leagues of them - searched for Captain Nemo for over 20,000 leagues and nights. 20,000 leagues later, they still hadn't found a trace of Captain Nemo, the man they called.. Ol' 20,000 Leagues Under The Sea! Captain Nemo: Noooooooo!!!!"
 * That makes it sound like she may or may not have been left drifting in deep space. Unfortunate implications, indeed!
 * In the Babylon 5 episode "Messages from Earth", one spaceship evades another by hiding in the atmosphere of Jupiter. The pressure on the exterior of the ship as it descends is given in "gravities", although gravities are a measure of acceleration and not pressure.
 * On the other hand, the net result of gravity's acceleration is perceived as pressure, since our senses are not equipped to accurately distinguish them. Not that there is much to distinguish, according to Einsteinian physics. Furthermore, when a pilot is trained to resist the extreme pressures caused by the sudden accelerations, these are spoken of as "G-Forces", as in, the forces you'd be facing in a planet with so-many-times the gravity of ours. It's obvious they were referring to that.
 * Humans have an entire sensory organ (the vestibular system) dedicated to sensing acceleration. We also do not confuse the perception of gravity and pressure at all (or people would not be able to walk inside airplanes, on high mountains, or hyperbaric chambers). Finally, to claim that there's no real distinction between acceleration and pressure in "Einsteinian physics" is simply ludicrous.
 * An example involving a unit made up for the show. Early on, it was decided that time on the station would be measured in cycles, possibly to avert Two of Your Earth Minutes. Unfortunately, none of the writers seemed to be able to agree on what a cycle was equivalent to (i.e. is it an hour? A day? A minute?). The idea was scrapped before the end of the first season. This was Lampshaded in a later episode when Ivanova stated that she was going to hold some troublemakers in custody for a Drazi cycle, after which the problems they were causing on the station would be sorted out. The Drazi she's explaining this to laughs, and points out that a cycle is a Drazi year, not a Drazi month.
 * In the documentary Forensic Files on Tru Tv, it is stated that a gunshot produced 120 decibels of sound, and that 70 decibels, or "about 60%" of that, went through to an adjacent room. The decibel scale is logarithmic, with 20 dB being equivalent to a factor of 10 increase in (Pa) amplitude, so 70 dB is really about 0.3% of 120 dB measuring by the raw pressure function.
 * Some sci-fi shows attempt to rectify such things by actually using the alien standards of time (see Microts). "Cycles" are a big favorite. Farscape is probably the most easily notable of these, with its microts, arns, and cycles. While a microt seems comparable to a second, an arn to an hour, and a cycle to a year, the actual usage definition seemed to be "An arn is longer than a microt and a cycle is a really long time." Since the scales for them were never actually defined, they were just as long as they needed to be for the show's purposes.
 * In "Through the Looking Glass", Crichton asks D'argo to wait five minutes or four hundred microts. Using this, one microt would be 0.75 seconds.
 * In the last season of Andromeda, Rommie refers to having been shut down for nanomillennia as if that's a long time. Nano- is a prefix meaning one billionth. One nanomillennium is about 31.5 seconds. One thousand nanomillennia is still less than one day. This might be considered a long time to a computer, but since Rommie had been offline for longer periods previously, that makes it a mistake.
 * The plot of one episode of The Middleman revolves around "4000 angstroms of Balthorium-G"; an angstrom is a unit of length, not mass or volume. Word of God is that the writers were perfectly aware of this, but did it anyway because they thought it sounded better.
 * Better yet the angstrom is equal to a tenth of a nanometre and is used because its about the diameter of an atom. So 4000 angstroms is only 0.4 micrometers, or less than a bacterium (which are between 1 and 10 microns) long.
 * The episode "Getting Barry Higher In The World" of Last of the Summer Wine features Seymour enlisting Wesley to build them a kite, but Wesley assumes that because Seymour's a pompous schoolteacher with a university education, all the figures on his plan are in metres rather than feet. The kite comes out over three times too large, but when it turns out to work as a hang-glider, Seymour claims I Meant to Do That.
 * In Boston Legal, opposing counsel threatened to "drag it out for light-years" if the main characters declined a settlement offer. (How far the pages will stretch?)
 * In an episode of the obscure Australian series Ship to Shore, a report (later revealed to be false) came in of a tsunami 30 feet (10 meters) high came in. Since some of the older islanders still used imperial units, the confusion snowballed to the point that the wave was supposedly 900 meters (3000 feet) high, and the islanders prepared for Armageddon.
 * Top Gear has a tendency to parody this. James May, being Good with Numbers, gives accurate conversions but refers to metric and imperial respectively as 'Roman Catholic' and 'Church of England'. Jeremy Clarkson on the other hand deadpans complete nonsense such as measuring torque by spreading his hands apart and saying 'about that much'.
 * An episode of Saturday Night Live hosted by Kelsy Grammer highlighted the confusion pointed out above regarding Twenty Thousand Leagues Under the Sea. Nemo calmly explains that he misspoke and he should have used fathoms to indicate depth. However, the other characters (even The Professor) end up describing every unit of measurement, even time, in terms of leagues, leading to Nemo being brought to the edge of madness before being dragged away by a giant squid.


 * Kamen Rider Fourze acknowledges the light-year issue in episode 12, when space Otaku Yuuki meets the school's chairman, a former astronaut. She says she's read his book "for light-years" and he immediately responds "Incorrect, light-years are a measure of distance". Rather than a mistake, however, it seems that Yuuki said this on purpose as a pretext to quote more of his book.
 * The CSI episode "Overload" includes, among its many atrocities against physics, Grissom confidently stating that "terminal velocity is 9.8 meters per second squared". High school physics should be enough to know that meters per second squared measure acceleration, not velocity (terminal or other).

Music

 * The Chris de Burgh song "A Spaceman Came Traveling" (a sci-fi interpretation of the first Christmas) explicitly refers to "light-years of time"; the writer not only makes the mistake but goes out of his way to wave it in the listener's face.
 * Joan Baez also made the light-years mistake in "Diamonds & Rust".

Radio

 * One of the many possible definitions of the diagonal in Mornington Crescent (specifically Archbald's Metropolitan Logorhythmic Progression) states "the diagonal shall be agreed to be any angle being agreed to be an angle between the angle of one degree and the angle of three hundred and fifty nine degrees ... Celsius".

Tabletop Games

 * As a "watts" example in reverse, GURPS sourcebooks insists on referring to kilojoules as "kilowatt-seconds". Although technically correct this is inelegant and confusing and proof that, even with the constant calls for them to abandon Imperial units, Steve Jackson Games shouldn't be let loose with SI.
 * They can't even keep that much straight. Centimeters get used from time to time, generally for weapons. More confusingly the abbreviation mps is used to talk about mile per second despite the fact that even people in the US think of it as meters per second.
 * That bit Steve Jackson at least gets correct. The abbreviation for meters per second is m/s and nothing else.
 * In at least one GURPS Traveller supplement, there's an extended sidebar on the difference between short tons, long tons, metric tons, water-displacement tons, and liquid hydrogen-displacement tons, all of which have some relevance in the setting. (A "100-ton ship", in Traveller, has a volume equal to 100 metric tons of LH 2.)

Video Games

 * Pokémon Red and Blue (as well as their remakes, FireRed and LeafGreen) referenced the light-year mistake with the Junior Trainer (Camper in the remakes) in Brock's gym, saying you're "light-years from facing Brock", but then acknowledging the mistake after you beat him.
 * Though really a distance unit works perfectly well there anyway... "You're miles away from facing Brock," anyone?
 * A possible explanation is a combination of Lost in Translation and Years Too Early.
 * An infamous example occurs in the lawnmowing minigame in No More Heroes, where "square meter" has been culturally "translated" into "acre" (roughly four thousand square meters).
 * This was of course mercilessly parodied in the Let's Play of the game. "You're a GIANT!"
 * In the various incarnations of Sim City, each tile square is also supposed to be 1 acre. Yet a tile square is only big enough for a single one-family house with hardly any yard, and it takes 4 adjacent tile squares to build even a small apartment building. This could be hand-waved as just being a representation of an acre of homes, until you realize THE ROADS ARE AN ACRE WIDE (actually over 60 meters wide).
 * Metroid Prime 2 manages to do this with its fictional measurement of time, the cycle. While the length of one cycle is never really stated, based on its use in the Space Pirate logbooks it would seem to be analogous to something between a few months and a year. However the Luminoth elder U'mos is described as being several centicycles old. They probably meant several hundred cycles, but the prefic centi means one-hundreth. Presumably the writers either weren't aware of the correct prefix, or just decided it sounded cooler than "hectocycle".
 * Fridge Brilliance: Adult moths have very short lifespans.
 * Averted in Zero Wing, of all things, which correctly puts A.D. before the number.

Western Animation

 * In one episode of Ed, Edd 'n' Eddy, Edd is working on an old radio, and realizes he'd mistaken a "fifteen-amp resistor" for another part. The problem is, amps are used to measure current, and the ohm is used to measure resistance. Resistors have power handling specifications too, but the unit for those is the watt, so that doesn't help.
 * In The Simpsons, Crazy Vaclav asserts that the car he's trying to sell will "do 300 hectares on a single tank of kerosene". The hectare is a measure of area, not distance.
 * Maybe his country is so poor the primary use of cars is to plow fields.
 * Grampa Simpson hates the metric system. "My car gets forty rods to the hogshead, and that's the way I likes it!"
 * If you're curious, that's approximately 120,000 liters per 100 km (0.002 miles per gallon). It must be a Hummer.
 * Or a crawler-transporter with a fuel leak (a fully functional crawler-transporter gets over 134 rods to the hogshead).
 * Mr. Burns has trouble with metric too, like in the episode he drops a weight marked "1000 grams" (just over 2 lbs) on Homer, to his minor annoyance, and then comments that it sounded much heavier when he ordered it.
 * In the episode where Bart goes to a gifted school, the kids con him out of his lunch by using units like picolitres to make it sound like they were offering more than they were.
 * In the beginning in one episode where Homer takes the kids to school, he uses a GPS which gives him the distances in meters. The confusion leads him through a construction zone.
 * In the Family Guy Star Wars movie Blue Harvest, Han Solo (Peter Griffin) claimed that the Millennium Falcon completed the Kessel Run in less than 12 parsecs. Luke (Chris Griffin) then tells an astonished Han than parsecs are a measure of distance not time.
 * In Transformers, (well, some series, anyway) "light-year" has been a unit of time.
 * Transformers has more units of time and distance than you can shake an exhaust pipe at. Most of them have never been explicitly defined, so any of them could be this trope.
 * An in-universe example: in Jimbo and the Jet Set, Jimbo's diminutive size is explained in the pilot episode as being the result of his designers mistakenly using centimetres instead of inches.
 * The French version of Thumbelina (Poucelina) has the Prince singing "Depuis des années-lumière, je n'attends que toi." ("For light-years, I've been waiting for you.").

Stand Up Comedy

 * Comedian Dr. Pete Ludovice has a routine where he suggests that men try to invoke Unit Confusion by reporting their penis size in nanometers.
 * There is a joke where a woman is telling her friend about a guy's penis and claims it's 700 nanometers. The other woman is astonished that it's so small, but the first woman replies that the penis is that red (the wavelength of the color red is 620–750 nm).

Webcomics

 * Early Schlock Mercenary strips have a nasty habit of referring to "watts of energy." Even as late as the third book, while it gets right that terawatt-nanoseconds would be a unit of energy (one more commonly known as the "kilojoule"), it treats it as though nanoseconds make it more incomprehensibly huge than "terawatt-hours" or, say, "terawatt-millennia."
 * It also plays with this trope with some things, such as freem, which are explained as the amount of pay a "Poliflorian Hypernetter earns about two thousand Freem during one Efrickalian work-week."
 * Comparison of two strips from the sixth book suggest he's confused diameter and radius.
 * Luke Surl shows a confusion of angle units on a chart.
 * Saturday Morning Breakfast Cereal shows self-explaining one.
 * Sluggy Freelance reminds us: pay attention to avoiding this while attempting a "Mission Impossible" Cable Drop. Ouch.
 * Chasing the Sunset had a pixie rather annoyed with Elven habit of measuring distance in "bowshots". Not unreasonably, as the first sample she happens to come across is a child's toy bow and the second is a ballista.

Web Original

 * Happens in universe in an SCP Foundation story. There is a big difference between mA and MA. Exactly how he managed to set the circuit to nine orders of magnitude lower than normal isn't given.

Real Life

 * In real life, the results of metric confusion can be very catastrophic and expensive, as when the Mars Surveyor '98 Orbiter crashed and was destroyed in 1999—the "small impulse" analysis software was giving data in pound-seconds, while the mission navigation team was expecting data in Newton-seconds.
 * A more down-to-Earth example (no pun intended) was the Gimli Glider. The Boeing 767 C-GAUN was the first Air Canada jet with metric instrumentation. There already had to be a measurement conversion because the refueling people on the ground measured kerosene in volume (gallons), while the aircraft measured fuel in weight (pounds or kilograms). The ground crew used the wrong conversion factor in calculating how many gallons to pump on the plane, leading the crew to think the plane had 22,000 kilograms of fuel on board, the amount needed for the flight, when it only had 22,000 pounds (9979 kg). The exact mass of fuel used is very important to aircraft and thus the gauge could not be replaced with a car style unitless gauge.
 * And in a supreme Guide Dang It (for the pilots/groundcrew)/WhatAnIdiot (for Air Canada) moment: nobody on the groundcrew or in the cockpit had been trained to do the proper conversion.
 * This was primarily due to the fact that this kind of thing used to be the job of the flight engineer. Unfortunately, computerized jets like the Boeing 767 have eliminated the need for that position, and apparently no one at Air Canada had stopped to consider who should take over the FE's duties.
 * Fortunately, due to supreme skill of the pilots and some good luck they managed to land the powerless plane (that's why it's commonly called Glider) on the former RCAF Gimli airbase, where one of the pilots was stationed back in his military days, and managed not to hit the drag races held on the runway they landed on, so nobody got hurt both on plane and on the ground.
 * Under normal circumstances, the fuel sensors would detect the actual amount of fuel loaded and that would prevent the problem. However, the fuel sensors were broken on that particular plane, which they actually knew, but this was not considered a serious enough fault to ground the plane. Without the sensors, the pilots depended on the FMC's calculated fuel weights to track fuel consumption during the flight, but because it had been given an incorrect starting weight, all its calculations were based on that and therefore also wrong.
 * Apparently a legal aversion: some people like to snark that 'pound' is weight (meaning the force of the object being pulled down), while 'kilogram' is mass, and the two aren't really the same. A notebook from the National Institute of Science and Technology specifically stated that as far as packaging, the two were pretty much the same. For most use on Earth, they are, anyway.
 * That's because the mass-to-weight ratio is a constant on Earth (or close enough: on average g = 9.81 m/s^2) meaning there is a linear correlation between mass and weight. On Earth it pretty much means the same thing then. However, move to environments with a different gravity (basically any other planet or moon, although the correlation will be linear on that world) or even space (where weight becomes meaningless) and you get a different story. Even worse: if you launch the object from a planet and into space, the gravity exerted on the ship becomes variable (inverse square law) meaning the correlation between mass and weight is no longer linear.
 * In scientific literature, you may find references to the "pound-mass" to refer to that concept, abbreviated "lbm", mainly because the Imperial unit of mass, the slug, is stupid. Whether that particular convention is followed or not, though, is hit or miss.
 * Made even more confusing since sometimes "lbf" is used for "pound-force", so sometimes you'll have "lb" (pound-force) and "lbm" (pound-mass), or "lb" (pound-mass) and "lbf" (pound-force) used in text. Best to just always stick with "lbf" and "lbm" and eliminate confusion.
 * Of course serious modern scientific literature should use the SI or units which are typically used in the field.
 * A standard measure of rocket engine performance is called the "Specific Impulse", which defined as the thrust per fuel consumption—or in traditional units: pounds (force) per pounds (mass) per second. Dividing out the pounds, Specific Impulse is therefore reported in "seconds". In SI units, it's Newtons/kilogram/second, which simplifies to m/s. In the basic rocket equations, specific impulse corresponds to the exit velocity of the exhaust.
 * Precious metals are measured in troy pounds (373 g), which are divided into 12 troy ounces (31 g) -- standard ("avoirdupois") pounds (454 g) are divided into 16 ounces (28 g). (Hence, an ounce of gold weighs more than an ounce of feathers, but a pound of gold weighs less than an pound of feathers.)
 * This video shows more examples of units with the same names but different values (e.g. short tons vs. long tons, statute miles vs. nautical miles vs. imperial nautical miles, etc.).
 * There's a story about a guy from Indonesia who got fired from a multinational company because he measured an ounce as 100 grams and a pound as 500 grams, which resulted in years of equipment malfunctions. It turned out that such units were once used in the Netherlands (called ons and pond), and although they're not formally used anymore today, the guy's home country, that is a former Dutch colony, keeps teaching it at schools.
 * Actually, they are still frequently used in the Netherlands.
 * This is because at the time the metric system was introduced into the Netherlands (in the 19th century), the ons and pond were the Dutch equivalents of the English ounce and pound, with approximately the same weights. They were redefined in metric terms to ease the transition, but unlike some other such measures never really fell out of use, though they really only get used for day-to-day measuring. Even today, many people ask for things like "an ounce of minced meat" when they want 100 grams. Very, very few of them will know that such an ounce was not always 100 grams — including translators, who often translate the English word "ounce" with either "ons" or "100 grams", making the quantity over three times as big at a stroke. Oh yeah, and don't even try finding a translator who knows how big a fluid ounce is.
 * At least in Germany, instead of saying "kilometre per hour", many people tend to use the abbreviation "km/h" in everyday conversation. But only the letters are said out loud, not the slash. If this unit wasn't so ubiquitous, you could easily mistake this for "kilometre multiplied by hour".
 * At least in Canada, people will sometimes use (or at least recognize) the military slang "klicks" as an abbreviation for "kilometre" and also use it as an abbreviation for "km/h", meaning there's a difference between "I have to go 20 klicks" and "I have to go at 20 klicks."
 * Ubiquitous before the introduction of metric measures, because every country had its own value. Napoleon Bonaparte was five foot two in French units, but five foot six in British units.
 * Carats and karats should not be confused; one is a unit of mass and the other is a unit of purity, respectively. Gems are measured in carats, with 1 carat = 0.2 grams. Gold is measured in karats, on a scale of up to 24.