If a work of fiction has a character who's good or supposedly good at mathematics, they will will inevitably screw something up somewhere down the line. When this happens, the project the group is working on comes crashing down, sometimes literally, and the mathematician will make a gesture of embarrassment and say, "I must have misplaced a decimal point."
This doesn't happen nearly as often in real life among mathematicians and scientists, since decimal notation is usually not the easiest way to write something. More common are radicals, fractions proper and improper, constants (such as pi or the natural base), and scientific notation.^{note } It does, however, happen at times—not very often (as traditionally one keeps track of things in terms of integer multiples of the lowest common currency, such as integers of cents instead of decimals of dollars)—in finance. It was an occupational hazard for engineers and scientists in the days of slide rules — see the Real Life section for details.
The most likely cause of the prevalence of this near iconic catchphrase for a mathematics error is a combination of Writers Cannot Do Math and Everybody Hates Mathematics. Misplacing decimal points is a characteristic of elementary school and middle school arithmetic. Thus, this will most commonly show up in works aimed at children, for the reasons that it doesn't take much for a kid to consider a term as advanced mathematics and that the writers themselves, being kids at heart, are the writers least likely to have taken advanced mathematics.
Subtrope of E = MC Hammer. Compare with Carry the One, the other arithmetic mistake common in fiction. See also Mismeasurement.
Examples:
- Some Carl's Jr./Hardee's ads have the announcer say that a sandwich costs $299, before he corrects it to $2.99.
- In Fantastic Four comics, Victor Von Doom misplaced a decimal when planning a clandestine scientific experiment. When his roomate Reed Richards tried to point it out to him, Doom went into a rage at the suggestion that he could have made a mistake and went ahead anyway. The experiment literally blew up in his face, scarring and forever embittering him against Richards, convinced that he must have changed the calculations in an attempt to sabotage him.
- In the Lucky Luke episode Outlaw, the Daltons (the original ones, not the cousins from the later stories) try to divide their loot among themselves. Having had no actual schooling, they fail horribly, turning a simple long division into a mathematical nightmare. Bob Dalton uses his gun to place a decimal point "to simplify things".
- At the end of Superman: Red Son Luthor's plan to destroy Superman and Take Over the World seems to have succeeded without a hitch... only for it to be revealed that Superman is still alive due to Luthor having misplaced a decimal point when he calculated Supes' density. However, due to Luthor having made Superman see the error of his ways as well as the world being a better place under Luthor's rule, he decides to let Luthor think he won and live the rest of his days in anonymity.
- Superman, as seen in the page image for Writers Cannot Do Math, thinks 20 times 16 times 10 equals 32,000 (the correct answer is 3,200).
- In ''The Flintstones'' (the 2016 comic book by DC Comics), local scientist Dr. Carl Sargon (Uh, No Celebrities Were Harmed thank you) calculates the trajectory of an asteroid with an abacus, concluding that it will collide with Earth in just a few days, destroying everything and killing everyone. The calculations were wrong because a couple of moths had been breeding in the abacus, and Sargon took them for a bead.
- The Ultimates: Banner considered that Captain America would barely have enough strength to talk. He awakes in full force and trash the floor with everyone. Banner admits that must have misplaced a decimal point somewhere.
- Invoked in all versions of The Producers. When begging Leo to not report his small scale embezzlement at the beginning, Max tells him he should just misplace a few decimals.
- The given cause of the plan's failure in Office Space was that Michael misplaced a decimal point, which results in far too much money being siphoned off. He claims that he always makes similar minor mistakes.
- More justified than most examples due to the simple fact that Michael wasn't doing math, he was writing a computer program that involved math. It's much easier to make a mistake like this under such conditions.
- Muppet movies:
- In The Muppet Movie, Kermit tries to buy a car at a rather disreputable dealership, but can't afford any of them, until Sweetums swats a fly which creates a decimal point in a price tag; the dealer finds himself selling a $1195 car for $11.95, after paying a $12 trade-in on the Muppets' old lemon.
- At the end of the telethon in The Muppets, the titular characters have raised $9,999,999 - one dollar short of the $10 million they need to save their theater. As Richman gloats, Fozzie bangs his head against the money counter in despair. The display flickers, rolls, and resets... revealing that the decimal point wasn't displaying correctly, and that they actually raised only $99,999.99.
- Kelly's Heroes uses this as a throwaway gag when Crapgame calculates the value of the Nazi Gold that Kelly is trying to steal. He initially comes up with $1.6 million, but two scenes later announces that he misplaced a decimal and it's actually $16 million.
- In Kill Me Again, the broke private eye played by Val Kilmer desperately needs some dough to get out of town. He checks his bank balance by phone and is agreeably surprised to find he has $732, then crestfallen to learn that the clerk meant $7.32.
- In James Belushi vehicle Made Men, Belushi plays Bill Menucci, a Con Man who stole twelve million dollars from the mob he worked for just before he became a witness for the Feds and was placed in witness protection. During the film his old mob, a group of local criminals, and the crooked local sheriff who's secretly having an affair with Bill's wife Debra all come after him for revenge, the money, or both. When the sheriff catches up with Bill and Debra he's especially pissed, because he and Debra were planning to steal the money from Bill, except Debra told him Bill had only taken $120,000. Bill finds that hilarious and teases Debra about leaving out zeroes and misplacing decimal places... right up until the sheriff abruptly shoots her in the head. That kind of sucks the humor right out of the situation.
- In Sing, the prize for the talent show was intended to be $1000 (composed mostly of various junk stuffed in a chest), but due to some antics with a Glass Eye, the secretary in charge of writing the ads ends up typing $100000, which attracts far more performers than usual and causes various other problems.
- In one of the Harold Shea stories by L. Sprague de Camp and Fletcher Pratt, where magic has a mathematical basis, one of the characters has to prove his magical skills by invoking a dragon. The guy accidentally shifts the decimal point two places to the right and summons 100 dragons. Luckily they're vegetarian.
- A later attempt to perfect the spell produces a dragon approximately 0.01 times the size of a normal dragon. It's ten inches long, breathes fire, has a sting in its tail...and was summoned into a cage made for the normal-sized dragon the characters were hoping to get. It promptly escapes by flying between the bars...and it's not happy.
- It is, however, adorable.
- A later attempt to perfect the spell produces a dragon approximately 0.01 times the size of a normal dragon. It's ten inches long, breathes fire, has a sting in its tail...and was summoned into a cage made for the normal-sized dragon the characters were hoping to get. It promptly escapes by flying between the bars...and it's not happy.
- In Unseen Academicals , the Bursar of Unseen University is said to regard the decimal point as a nuisance. Inevitably, this leads to Ponder Stibbons taking over his responsibilities. Although being utterly mad may have more to do with it (not that that stopped him before) as he had previously been shown to be very mathematically adept.
- As of The Science of Discworld, the Bursar is now too mathematically adept to bother with decimal points, or real numbers. "Of course, he was a natural mathematician, and one thing a natural mathematician wants is to get away from actual damn sums as quickly as possible and slide into those bright sunny uplands where everything is explained by letters in a foreign alphabet".
- It might be noted that the bursar of a university is in charge of the finances. Not exactly a position for someone who can't be bothered with decimal points (or someone who is barking mad, if it comes to that).
- In The Purchase of the North Pole by Jules Verne, the antagonists' plan is doomed to fail from the very beginning, because, as it is revealed in the end, the mathematician responsible for it was interrupted while writing down Earth's perimeter, which caused him to effectively move the decimal point three places to the left.
- In "Homo Sol", a short story by Isaac Asimov, an alien scientist mentions an occasion when one of his students thought he'd disproved a longstanding theorem. It turned out he'd misplaced a decimal point in an exponent, meaning his result was 10000000000 times too large.
- In the Star Wars Expanded Universe X-Wing: Mercy Kill, main character Voort saBinring is a genius mathematician using his skill for strategy and tactics both in starship combat and in commando situations. He develops one of these vaudevillian schemes to capture the villain (of course, the reader is not told the details because otherwise the scheme would fail) but when executing it, he decides he had "missed a variable" when they kidnapped a person who was supposed to be kidnapped by other people to attract attention.
- In The Sack, Senator Horrigan does this twice, first when he multiplies 100,000 by 120, then when he multiplies 100,000 by 180. He gets 120 and 180 million, respectively.
- In Mystery Science Theater 3000: The Undead, Tom Servo misses a single question on the tests given to the cast by the Brain Guys (which everyone else fails horrendously but Gypsy, and it's suggested via Overly Narrow Superlative that she'd have failed if they'd been able to decide on a control set) for this reason.
- Played with in Red Dwarf: White Hole, Holly's IQ has been significantly increased (to 12000) in exchange for exponentially reducing her lifespan. When looking at her new lifespan, the screen displays 345 before she realizes "The decimal point, where's the decimal point?" She then discovers that she has 3.41 minutes left to live.
- Another episode featuring another hologram named Queeg actually Holly pulling a prank on the crew insults Holly's IQ in this manner.
- In Doctor Who episode "The Pyramid at the End of the World", a misplaced decimal point during a biochemical experiment results in a highly-voracious bacteria that threatens to wipe out all life on Earth within a year, if it gets out into the atmosphere. Why was it misplaced? Because one of the scientists accidentally broke her reading glasses, and her colleague had a bad hangover.
- In the WKRP in Cincinnati episode "The Contest Nobody Could Win", Johnny Fever misreads the prize for a radio contest as $5,000 instead of $50.
- The Wire: When Major Colvin is at COMSTAT presenting his official numbers showing a 14% drop in felonies in the Western District as a result of his Hamsterdam, Bill Rawls is skeptical of Colvin's numbers and says he's going to have them looked over to make sure Colvin didn't leave out a decimal point between the 1 and the 4.
- The Partridge Family: In "Forgive Us Our Debits," Shirley buys a $29 cuckoo clock and receives a bill for $290 thanks to a computer glitch. When she goes back to the store to get it fixed, repo men show up at the house threatening to take her furniture if she doesn't pay the $2900 she owes.
- Bentley sometimes steers you directly into security in Sly Cooper because of this or Carry the One. As a rule, if he says his calculations are correct, he's wrong.
- Plato's ten-fold error, mentioned below in Real Life, shows up as a plot point in Indiana Jones and the Fate of Atlantis, both in regards to the distances Plato gave, as well as the number of Orichalcum beads required to make the Atlantean ascension device work properly.
- In the early days of City of Heroes, the smoke bomb in the /devices powerset reduced foes' perception to zero, allowing /devices blasters to kill foes with impunity. This turned out to be due to a misplaced decimal in the magnitude of the debuff (50% instead of 5%), and was subsequently corrected.
- Discussed in The Answer of Persona 3. When speculating about why the researchers were wrong in predicting that the destruction of Tartarus would also cause the Abyss of Time to cease to be, and that said Abyss would have no Shadows, Junpei makes the following retort:
Junpei: Did they forget a decimal place or somethin'?
- In Touhou, Mountain of Faith has a bug where Marisa Kirisame's lasers are ten times as powerful than they are supposed to be.
- In Questionable Content, Dora panics briefly because of one of these. Hannelore comforts her with that her Corrupt Corporate Executive mother made the same mistake, causing the Argentinian economy to collapse.
- In Leftover Soup, Max's older brother Mark died because he misplaced a decimal point during training and caused an accident.
- In Melonpool, technical guy Ralphie says he "must've missed a decimal point somewhere" during one of the time-travel paradox plots. It makes some sense in the context.
- In Ed, Edd n Eddy, Edd seems to have made this error on a few occasions.
- In The Amazing World of Gumball episode "The Check", Louis gives the kids a check for $5000. Just as they are about to cash it at the end of the episode, Louis arrives and announces that he forgot to add the decimal point, making the check $50 instead.
- There's an urban legend that spinach is said to have a high iron content because once upon a time, someone misplaced the decimal point, and the figures on modern nutrition labels are because every single person in every nutritional agency in the world since is just making computations based on that figure, not doing tests of any kind. This is, of course, false.
- Supposedly, the myth of Atlantis got started because Plato did the equivalent of misplacing a decimal point in his memoirs — if he had recorded the east-west distance as exactly one-tenth of what he described, it would point directly to Crete. Decimals hadn't been invented then, but there was confusion about the meaning of a particular hieroglyphic, mistaking 100 for 1000.
- A Calorie is 1000 times as much energy as a calorie. That is, the "Calorie" seen in American nutrition charts is actually a kilocalorie. This has to rank among the stupidest units of measurement (i.e naming conventions) that exist. It seems deliberately designed to confuse and lead to errors.
- Isaac Asimov also mocked this concept in one of his scientific essays, and proceeded to use kilocalories (abbreviated kcal) for the remainder of his calculations.
- Which has led to the fallacy: "You need one calorie to heat up 1 g water (or another drink) one degree celsius. One liter of a softdrink equals roughly 1000 g, but contains less than 500 calories. So if you drink a cold softdrink, your body will burn more calories than the drink contains to heat it up to body temperature!" Unfortunately, the drink really contains less than 500,000 calories.
- Something similar can happen whenever someone mentions a "billion" of something. In the United States it means a thousand millions, in other countries it means a million millions (a thousand milliards). This is presumably one reason that Stephen Hawking used the word "million" in multiples throughout his book A Brief History Of Time to describe large numbers. The British House of Commons actually passed a law in the 1970s to officially get rid of the (until then) common British term "milliard" because the City of London as a global center of finance could not risk milliards and billions getting mixed up.
- According to this blog post, Verizon in 2006 fell victim to bad unit conversion, advertising a data plan as 0.002 cents ($0.00002) per kilobyte but actually charging $0.002 in dollars. They eventually refunded the blogger in question according to the advertised rate.
- This was probably much more common in the past, as slide rules don't give you a decimal point except in a very limited range of calculations. You're expected to place it yourself from context. (It's how a simple sliding scale manages to be so versatile — you make exactly the same motions to multiply something by 42, or 4.2, or 42,000.)
- It was common to accompany a complex accurate slide rule calculation with a 'back-of-the-envelope' recalculation. Use the slide rule to multiply 42.13 by 0.00972, and compare to multiplying 40 by 0.01 by hand as a check (except that a real calculation would likely have many more operations than a single multiplication.)
- This can happen nowadays as well if you use the scientific notation and calculate the exponent separately. for example 7*10^5 times 3*10^7 can be simplified to 7*3*10^(5+7) or 21*10^12 adjusting the mantissa yields 2.1*10^13; forgetting to increment the exponent while adjusting will lead to the trope.
- In a brilliant aversion of Stupid Jetpack Hitler German physicist Walther Bothe, who worked at their atomic bomb project, did exactly this when he determined the neutron free-flight length in graphite to be almost ten times smaller than it was in reality, forcing German nuclear project to use much more expensive and finicky heavy water instead of cheap and easily available graphite.
- A pilot on a flight from Maraba to Belem, on reading his flight plan, found his course was written as "0270". He interpreted it as 270 degrees (west), but in the flight plan it was assumed there was a decimal before the last digit, so the course was actually 027.0 degrees (roughly north-north-east). The plane ended up crashing in the jungle hundreds of miles from Belem when the fuel finally ran out. 13 people died.
- Some languages use decimal commas, so even a correct number can cause confusion to translators (compare 1,255 to 1.255).