If a work of fiction has a character who's good or supposedly good at mathematics, he or she 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]]Though technically, using the wrong exponent here is equivalent to misplacing the decimal point, sometimes over many orders of magnitude[[/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.

The most likely cause of the prevalence of this near iconic catchphrase for a mathematics error is a combination of WritersCannotDoMath and EverybodyHatesMathematics. 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 EEqualsMCHammer. Compare with CarryTheOne, the other arithmetic mistake common in fiction. See also {{Mismeasurement}}.
----
!!Examples:

[[foldercontrol]]

[[folder: Comic Books ]]

* In ''ComicBook/FantasticFour'' comics, [[DoctorDoom 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 ''ComicBook/LuckyLuke'' episode ''Outlaw'', the Daltons (the original ones, not the nephews from the later stories) try to divide their loot among themselves. Having had no actual schooling, they [[EpicFail 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 ''SupermanRedSon'' [[spoiler:Luthor's plan to destroy Superman and TakeOverTheWorld 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 WritersCannotDoMath, thinks 20 times 16 times 10 equals 32,000 (the correct answer is 3,200).

[[/folder]]

[[folder: Film ]]

* Invoked in all versions of ''Film/TheProducers''. 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 ''OfficeSpace'' 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 ''TheMuppetMovie'', Kermit tries to buy a car at a [[HonestJohnsDealership 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' [[TheAllegedCar old lemon]].
** At the end of the telethon in ''Film/TheMuppets'', 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.
* ''Film/KellysHeroes'' uses this as a throwaway gag when [[TheScrounger Crapgame]] calculates the value of the NaziGold that [[TheHero 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 ''Film/KillMeAgain'', 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.

[[/folder]]

[[folder: Literature ]]

* In one of the ''HaroldShea'' stories by Creator/LSpragueDeCamp 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''.
* In ''Discworld/UnseenAcademicals'' , 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 ''Discworld/TheScienceOfDiscworld'', 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".
* In ''The Purchase of the North Pole'' by Creator/JulesVerne, 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 Creator/IsaacAsimov, 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 Franchise/StarWarsExpandedUniverse ''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 to be kidnapped by other people to attract attention.

[[/folder]]

[[folder: Live Action TV ]]

* In ''Series/MysteryScienceTheater3000: TheUndead'', 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 OverlyNarrowSuperlative that she'd have failed if they'd been able to decide on a control set) for this reason.
* Played with in ''Series/RedDwarf: [[SwirlyEnergyThingy White Hole]]'', Holy's IQ has been significantly increased (to 12000) in exchange for [[ExplosiveOverclocking 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 [[DrillSergeantNasty Queeg]] insults Holy's IQ in this manner.
---> '''Queeg''': It has a six in it, but it's not six thousand.
---> '''Lister''': Well what is it?
---> '''Queeg''': [[DeadpanSnarker Six.]]

[[/folder]]

[[folder: Video Games ]]

* Bentley sometimes steers you directly into security in ''Franchise/SlyCooper'' because of this or CarryTheOne. As a rule, [[IfMyCalculationsAreCorrect if he says his calculations are correct]], [[TemptingFate he's wrong]].
* Plato's ten-fold error, mentioned below in RealLife, shows up as a plot point in ''VideoGame/IndianaJonesAndTheFateOfAtlantis'', both in regards to the distances Plato gave, [[spoiler: as well as the number of [[AppliedPhlebotinum Orichalcum beads]] required to make the Atlantean [[UpgradeArtifact ascension device]] work properly.]]
* In the early days of ''VideoGame/CityOfHeroes'', 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 ''VideoGame/{{Persona 3}}''. When speculating about why the researchers were wrong in predicting that [[spoiler: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.

[[/folder]]

[[folder: Webcomics ]]

* In ''Webcomic/QuestionableContent'', [[http://questionablecontent.net/view.php?comic=1458 Dora panics briefly because of one of these.]] Hannelore comforts her with that her CorruptCorporateExecutive mother made the same mistake, causing the Argentinian economy to collapse.
* In ''Webcomic/LeftoverSoup'', Max's older brother Mark died because [[http://leftoversoup.com/archive.php?num=358 he misplaced a decimal point during training and caused an accident.]]
* In ''Webcomic/{{Melonpool}}'', technical guy Ralphie says he [[http://www.melonpool.com/?p=680 "must've missed a decimal point somewhere"]] during one of the time-travel paradox plots. It makes some sense in the context.

[[/folder]]

[[folder: Western Animation ]]

* In ''WesternAnimation/EdEddNEddy'', Edd seems to have made this error on a few occasions.

[[/folder]]

[[folder: Real Life ]]

* 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 [[IdiotPlot 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, [[http://www5.in.tum.de/~huckle/Sutton_Spinach_Iron_and_Popeye_March_2010.pdf false]].
* Supposedly, the myth of Atlantis got started because Creator/{{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. [[http://en.wikipedia.org/wiki/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.
** Creator/IsaacAsimov 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).
* It seems the people at Verizon have this problem. [[http://verizonmath.blogspot.com/2006/12/verizon-doesnt-know-dollars-from-cents.html Misplacing the decimal can be a real problem when you're talking about money.]]
* 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.)
** This can happen nowadays as well if you use the [[http://en.wikipedia.org/wiki/Scientific_notation 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 StupidJetpackHitler 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.

[[/folder]]
----