## How do you say 100,000,000,000,000,000,000 in words?

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One of the answers in a reading exercise in my class today was:

• 100,000,000,000,000,000,000

... which was the value of the highest denomination note ever issued. It was a 100,000,000,000,000,000,000 Pengo note, issued by Hungary in 1946. The students in my class wanted to know how they could say that last sentence in words in English. So I told them that I didn't know. They pointed out to me that if they wanted to read this bit of writing to someone they'd want to be able to use the words. Fair point.

Can anyone help?

[In case you're interested, a 100,000,000,000,000,000,000 Pengo note was worth about $0.20!] 7 wolfram alpha is great for this sort of thing: http://www.wolframalpha.com/input/?i=100%2C000%2C000%2C000%2C000%2C000%2C000+ – Zevan – 2015-06-23T21:54:38.520 For future word/number conversions: http://www.calculatorsoup.com/calculators/conversions/numberstowords.php – MikeTheLiar – 2015-06-24T14:31:30.797 Additional note: the currency was called pengő, not Pengo. – Janus Bahs Jacquet – 2015-06-24T17:58:17.247 This map shows that the "long scale" isn't really used in English-speaking countries any more. My understanding is that the reason that Canada is purple in this map is because the French speakers there use the "long scale", but the English speakers use the "short scale". – Dawood ibn Kareem – 2015-06-27T03:47:49.030 ## Answers 121 It's: one hundred quintillion or: a hundred quintillion ## The words for very large numbers If you're wondering how to form other huge numbers like this, here's the pattern: A thousand thousands is a million: 1,000,000. A thousand millions is a billion: 1,000,000,000. A thousand billions is a trillion: 1,000,000,000,000. A thousand trillions is a quadrillion: 1,000,000,000,000,000. A thousand quadrillions is a quintillion: 1,000,000,000,000,000,000. A thousand quintillions is a sextillion: 1,000,000,000,000,000,000,000. And so on. The part before -illion is the Latin prefix for the number of times you went through the process of multiplying by a thousand. So, you can continue to septillion, octillion, nonillion, decillion, undecillion, duodecillion, and so on forever. Within the scale defined by one of these huge units, you multiply by a number from 1 to 999 in the usual manner, by putting the multiplier ahead of the unit, and you add smaller numbers by putting them after the unit, in the same manner as for thousands: 215,002 is "two hundred fifteen thousand and two". 215,000,000,000,000,000,002 is "two hundred fifteen quintillion and two". The usual customs for "hundred" apply: 123,456,100,000,000,000,000 is "a hundred and twenty-three quintillion four hundred and fifty-six quadrillion one hundred trillion" or "one hundred twenty-three quintillion four hundred fifty-six quadrillion one hundred trillion", or other variations, the same as for hundreds of thousands. ## Exponents When you work with these numbers on a daily basis like I do,* you soon find that they become rather unwieldy, at least until you get up to a centillion pengős. In the physical sciences, if not in economics, one normally writes and pronounces these numbers using powers of ten. A quintillion is 1018, which you pronounce like this: Ten to the eighteenth power. Ten to the eighteenth. [for short] Ten to the eighteen. [even shorter] In scientific notation, you always choose an exponent large enough so the multiplier has one digit to the left of the decimal point, like this: 2.15 ⨉ 1017. That's pronounced: 215 quadrillion is "two point one five times ten to the seventeenth." If the multiplier is exactly 1, you can omit it in speech. So: 100,000,000,000,000,000,000 is a hundred quintillion, or ten to the twentieth power. ## Does anybody really say “quintillion”? “Quintillion” is an obscure word, though not much more obscure than “quadrillion”, which often gets totted out when government budgets and monetary inflation make news. A fluent speaker can guess it from the pattern of “billion”, “trillion”, etc. Here are a few samples to illustrate typical contexts where people really use it to communicate (that is, not just to talk about words for huge numbers, which might be its most frequent use): Government budgets: “For instance, the expected state income for oil and gas was reduced from 99,591 quintillion rupiah (about 9 billion euro) to 72,930 quintillion rupiah.” Pop science: “The quantum simulation of the 69 electrons must specify all possible 600 quintillion states simultaneously.” Bizarre religious tracts: “When this universe collapses in 70–100 billion years from now Jesus has given Kush a Quintillion universes like the one we live in as its territory forever. That is our promised land.” Very low probabilities resulting from calculations: “Using FBI statistics, Schoon calculated that the DNA profile at issue would be found in 1 in 2.7 quintillion African-Americans, 1 in 52 quintillion Caucasians and 1 in 260 quintillion Hispanic unrelated individuals.” (This is from a U.S. appellate court opinion.) Often when “quintillion” appears in print, it’s accompanied with an explanation. Usually when I’ve seen it used without explanation, it’s been in the context of economics. Presumably that crowd is well accustomed to talking about vast sums of money. ## Long scale and short scale Notice that in the Indonesian budget described above, “quintillion” occurs with multipliers greater than 999. That suggests that they're following the “long scale” system, in which each successive ‑illion is a million times greater than the previous one. That's an older usage, now nonstandard in English in all countries, but some people still use it, especially in countries like Indonesia where the dominant language follows the long-scale system. See kasperd's answer for more about that. *Just kidding. When you get up to a centillion pengős, you're talkin’ real money. 2but some system uses millions as the word counting basis: eg 1 million (10^6) is a million, a million million (ie 2 of them or 10^12) is a billion, a million million million (3 of them or 10^18) is a trillion, etc. I'm not sure which is more common but IIRC British use my definition of billion, and Americans use yours (ie a thousand times multiplier for each word step) – gbjbaanb – 2015-06-24T13:31:31.953 The classical usage is for naming structures to contain 1 order of magnitude greater quantity than the prevous, hence 999 thousand is in thopusands and 1000 thousand is a million, using the classical naming rules, it is 100 trillion. The rules above are in effect the american financial scales. Shall post a better answer later on:) – GMasucci – 2015-06-24T14:15:55.763 FWIW, my English instructor always stated the word and should only separate wholes and decimals: "One Thousand Two Hundred Fifteen and twelve-one-hundredths" or some such. Is that regional, and I've been wrong this whole time, or grammatically correct, yet uncommon? – phyrfox – 2015-06-24T16:11:34.500 @phyrfox "And" can precede whatever comes after "hundred". For example, "a hundred and twenty-five". Here is a question about it. Instructors often make things seem more rule-like than they really are, because of the difficulties of running an orderly class. – Ben Kovitz – 2015-06-24T18:17:57.280 @BenKovitz Thanks for the link. I actually realized why "and" generally shouldn't be used after reading that link, because it is technically grammatically incorrect. I also realize that (almost) nobody cares, because the intent is the same, meaning it is well understood by most English-speaking people. At any rate, I discovered something new today, so thanks for that. – phyrfox – 2015-06-24T18:23:45.543 @BenKovitz I never questioned it until I saw the logic behind the argument. Unless you're teaching arithmetic, or want to draw a distinction between two groups, you wouldn't normally say "I have 5 apples and 2 apples," so extending this logic, you "wouldn't" normally say "I have 100 (apples) and 2 apples." I'm not here to start a war, by the way, I'm okay with either method, but I finally saw the logic behind the "and problem." Also, "I have one hundred and two dollars and fifty-six cents." It feels wrong, because there's two groups (dollars, cents), not three. – phyrfox – 2015-06-24T18:53:20.840 @phyrfox No war needed, of course, but there is indeed controversy about this, like a lot of things in English; this ELU question has a little more info. In the UK, I think people often perceive omitting the "and" after "hundred" as an error. Even growing up the U.S., omitting the "and" sounded like an error to me, too. I've heard it enough as an adult that it no longer sounds like an error, but to my ear it still sounds like "paragraph jockey" usage. – Ben Kovitz – 2015-06-24T18:57:12.683 In Hungary at this time, the long scale was in use. – Nicholas Shanks – 2015-06-25T17:08:44.997 I realize this question is more than academic, but wouldn't it be "dodecillion" instead of "duodecillion" when it comes to twelve? As in "dodecanol"? – Sir Jane – 2015-06-26T12:40:52.140 2 @Gloria The custom in chemistry is to use Greek prefixes: mono-, di-, tri-, tetra-, penta- and so on (except for 9 and 11 (!) and except for 1–4 for carbon chains). A few of these prefixes are the same or similar in both sequences, though: tri-/tri-, octo-/octo-, dec-/deca-, undec-/hendeca-, duodec-/dodeca- etc. Solid shapes in geometry also follow Greek: tetrahedon, dodecahedron, etc. This page has lots more information about the sequences and when each is customarily used in English. – Ben Kovitz – 2015-06-26T14:42:18.027 There is at least one area where the word undecillion is actually used: the new Internet addressing protocol IPv6 has 128 bits, which allows approximately 3.2x10^38 different IP addresses. Spoken, that is about 320 undecillion IP addresses. – Kevin Keane – 2015-06-29T04:00:07.823 In British and international English, omitting the “and” is an error. Plain and simple. 215,002 is "two hundred AND fifteen thousand and two". – Chris Melville – 2020-05-25T09:01:50.217 35 Wikipedia lists large scale numbers here. As only the 10x with x= multiples of 3 get their own names, you read 100,000,000,000,000,000,000 as 100 * 1018, so this is • 100 quintillion in American and British English and • 100 trillion in most (non-English speaking) other places. (Practical approach: The different naming patterns for large numbers obviously can lead to misunderstandings internationally, expressing large numbers in the unambiguous format of X * 10y may be preferrable in these cases. ) Some additional information on the Hungarian money for your students: Wikipedia (in the same article as above) claims that the highest note issued was actually 10 times as "valuable": The highest numerical value banknote ever printed was a note for 1 sextillion pengő (1021 or 1 milliard bilpengő as printed) printed in Hungary in 1946. 13In what parts of the world is long scale numbering still in use among English-speakers? – choster – 2015-06-23T14:30:51.153 13@choster Problem is that these terms are very similar or even identical in many languages. If I (German) read "billion" I think "1,000,000,000,000" whereas my (US) counterpart means "1,000,000,000". It's always a good idea to keep this in mind, these misunderstandings are surprisingly common. Being wrong by a factor of 1,000 (or even more for trillion etc.) is no picknick. – Stephie – 2015-06-23T14:48:11.177 4And as Britain switched from long to short scale in the 1970s, it's good to double-check for older documents which scale was used... – Stephie – 2015-06-23T15:12:01.797 9"One hundred million million millions" would be safe. And lots of people wouldn't know what 100 quintillion is. – gnasher729 – 2015-06-23T16:00:46.830 3 @gnasher729 "Quintillion" is obscure word, but not so unheard-of that it doesn't occasionally make it into New York Times headlines. Please see comments here for why "million million millions" is at least as unsafe, and my answer for some more detail about the obscurity of "quintillion". – Ben Kovitz – 2015-06-23T19:00:56.763 1I think I prefer the word "sextillion"... for obvious reasons. – Omegacron – 2015-06-23T19:19:01.660 I (in my late 40's) learned the long scale first when I was a child and was taught that the short scale was "American". Even in the early 80's people argued about it. – Francis Davey – 2015-06-24T06:35:32.120 1Presumably some English speakers in India would just call this one hundred gulshan. – slebetman – 2015-06-24T08:47:37.613 13 You can call it a hundred million million million. The three million in a row can be a bit confusing, which is why the word trillion was invented. Trillion is a contraction of tri (meaning three) and million. That would make the name a hundred trillion. Unfortunately some languages including English have redefined the word trillion to mean 1 000 000 000 000. And simultaneously redefined the word quintillion to mean 1 000 000 000 000 000 000. This means there are now two different names for that number, and each name has a totally different meaning to somebody else. Which means you can use either word and people will know that you are talking about a big number, but nobody will be quite sure which number you are talking about. You could also call it a tenth of a trilliard which to the best of my knowledge is unambiguous. If it is important that the person you are speaking to know exactly which big number you have in mind, it is best to use mathematical terms in which case the name will be ten to the twentieth (power). 6Indeed it is a tragedy that "billion" came to mean "a thousand millions", since the bis- prefix is Latin for "twice" and "billion" was a nice way of saying "a million, twice". (I understand the contracted prefix in "trillion" to be ter-, which is Latin for "thrice", but that's just me.) Do you know how the Americans came to use -illion for thousand (sort of)? I've never read anything on how it happened. – Ben Kovitz – 2015-06-23T19:38:35.487 1 @BenKovitz Wikipedia mentions trimillion and tryllion as older spellings if trillion. I don't know how the Americans came up with a different meaning. I am guessing it started as a mistake and there may have been nobody around to notice and correct the mistake at first. – kasperd – 2015-06-23T22:12:33.237 3 I found this in the OED: "According to Littré, it was only in the middle of the 17th c. that the ‘erroneous’ custom was established of dividing series of figures above a million into groups of three, and calling a thousand millions a billion, and a million millions a trillion, an entire perversion of the nomenclature of Chuquet and De la Roche." Littré seems to be the author of a French dictionary. – Ben Kovitz – 2015-06-24T01:38:42.640 4 Found something: Florian Cajori says it grew out of the 17th-century French custom of dividing numbers into periods of three digits instead of six. The French apparently "perverted" the excellent terminology they invented, and the U.S. copied. – Ben Kovitz – 2015-06-24T01:43:20.177 @BenKovitz Looks like there is enough to say about the history of those words that it would make for a nice Q&A on its own. I wonder if it might have been asked and answered already. – kasperd – 2015-06-24T07:10:16.453 "How was long-scale numeration lost?" might indeed make a good question on http://elu.stackexchange.com. As far as I can tell, no one has yet asked it. – Ben Kovitz – 2015-06-24T18:06:38.733 3@BenKovitz Littré wrote a dictionary that still defines the standards of writing good literature in French (though it was updated a bit since). Now the dictionary itself goes by the name of Littré, like "I checked that up in the Littré". Back to the issue, in France and most of Europe, the long scale is in use nowadays, even though, like you said, the short scale was used for a while back in the 17/18th centuries. It happens to be the time at which most of the settlers crossed to America, so I wouldn't be surprised it just stayed when Europe went back to long scale. – spectras – 2015-06-27T15:38:58.573 11 one hundred quintillion  You can try all sorts of numbers on a site like this: http://saythenumber.com/?n=k2Y 2This doesn't answer the question. Stack Exchange sites aim to be a repository of information, not a link farm. – David Richerby – 2015-06-23T18:47:48.297 6I know disagreement will be useless from my part, but this directly answers the question. – engspeaker – 2015-06-23T19:03:27.087 1 The answer to this question is a wiki link: Names of large numbers. – Mazura – 2015-06-25T02:11:14.477 3@Mazura: Links are never answers. Period. – Lightness Races in Orbit – 2015-06-27T19:55:08.790 10 Outside of mathematics, it is also common to read such a number as "one hundred million million million". 8Common where? I have very seldom heard or read this sort of stacking, beyond the occasional poetic use, e.g. "a thousand thousand warriors". – Nathan Tuggy – 2015-06-23T15:25:58.113 I also have very seldom heard of 100 quintillion, in mathematics or elsewhere. And I would never in a million million years trust the person using that number to get it right. – gnasher729 – 2015-06-23T16:02:34.253 3 @NathanTuggy You should get out more, then! ;-) – Araucaria - Not here any more. – 2015-06-23T16:10:45.333 Prof. Brian Cox often says things like that, sometimes with even more repetitions of "million". I think it's questionable whether the person listening actually receives the number communicated to them correctly in many cases, but it certainly has an effect that I don't think is purely aesthetic. Those who really, really, [slight pause] really like his voice and his cadence when speaking might consider it poetic, though ;-) – Steve Jessop – 2015-06-23T18:02:15.320 3 I've tried using the "million million …" approach with native AmE speakers, and the result has always been instant confusion. The problem is that beyond a couple "million"s, the listener's short-term memory easily loses track. But if you want an amusing tidbit to add to this answer, a usage of "billion billion" occurs near the beginning of Monty Python's "Science Fiction Sketch". – Ben Kovitz – 2015-06-23T18:03:21.187 The same trick can be done with "quintillion", by the way. This creationist tract says that an unnamed astronomer thinks the odds of a life-friendly planet's existing are "one in one thousand quintillion quintillion quintillion quintillion quintillion quintillion quintillion". – Ben Kovitz – 2015-06-23T18:11:13.303 3Saying "million" three times in a row can be a bit confusing which is why another word was constructed for this. Taking tri (which means three) and million contracted to trillion simply substitutes for saying million three times in a row. Unfortunately somebody decided to redefine the word trillion to mean billion. Which leaves us mostly with ambiguous ways to express that number. – kasperd – 2015-06-23T18:13:27.803 3 @NathanTuggy It's common in the British media. "... objects containing a million million atoms or more" (BBC); "1,000,000,000, 000,000,000,000,000,000. A thousand million million million million!" (The Guardian); "contributing approximately £12 million million to GDP" (Financial Times). – David Richerby – 2015-06-23T19:36:28.587 I'd say this is more common than the other answer of quintillion, as if you say '100 million million million' everyone will know what you mean, quintillion will be understood by very few – tony – 2015-06-25T14:08:03.033 2No-one understands either 100 million million million, nor one hundred quntillion. These numbers are too big to comprehend. – GreenAsJade – 2015-06-26T23:46:16.850 It's not particularly big. It's the number of bytes that you can store on 100 million reasonably large hard drives, so about the number of bits that all US citizens can store on their hard drives. – gnasher729 – 2016-06-11T19:03:45.883 In case the other comments haven't made it clear, this usage is not at all common in AmE, which is probably why one commenter noted confusion when using this nomenclature with native AmE speakers. AmE speakers would not talk about "15 million million" in regards to the GDP, for example, but would rather talk about "15 trillion." – reirab – 2018-12-28T19:25:35.760 8 As others before me have said, there are names for very large numbers, but these are quite esoteric and people may not know what you're talking about. These names aren't like "thousand" or "million", unless you've sat down with a list and learned them, you probably won't know them. They just don't come up in day-to-day use. In my experience, they tend to be like English terms of venery. Some people have learned that you can say "a parliament of owls" to refer to a group of owls, but it's very rare to see these words in normal conversation. Usually they come up in quizzes and as a matter of trivia or curiosity. I would be more likely to just read it, like englishimprover.com, as "a hundred million million million" - or "a hundred billion billion". Using google ngram, we can compare how commonly these terms are used. It seems that quintillion is gaining in popularity, but million million million still outnumbers it Billion billion vastly outnumbers quintillion A word like duodecillion is a rare thing indeed. My spell checker doesn't know it Of course, in a scientific, or economic, context, you could convert this into what's called "standard form". For simplicity and readability, most people working with large numbers stop using their names after a point - and, indeed, stop writing them out in digits. As Ben Kovitz says, you could simply read this as "one times ten to the twenty" (that's how we'd do it here in Britain), or "one hundred times ten to the eighteen" (less likely). We could also re-write it: 1 x 1020. 8 100,000,000,000,000,000,000 is 100×1018 or 100×103+(3×5), so this is "one/a hundred quintillion" to most English speakers around the world today (including those in the United States, United Kingdom, Canada, Australia, New Zealand, India, Pakistan, and Nigeria). While many native English speakers will not know the rarely seen word, at least in the U.S. it is generally used when appropriate in mass media (e.g. newspaper articles), though often followed by a clarifying aside like "that's 100 million million millions" when the order is above "trillion". If you're teaching students, it's the perfect sort of word to introduce for the sake of expanding vocabulary, though I strongly recommend you connect it to the Latin prefixes to help it stick. Due to the difference between the long and short scales, beware "false friend" words in other languages—especially French, in which the cognates are spelled identically—and when reading historical English text (written as recently as the 1970's in the case of the UK). For example, the French "100 trillion" translates to the English "100 quintillion", not the English "100 trillion". Due to the possible confusion caused by the two scales, when clarity is particularly important and/or the audience particularly diverse, it's a good idea to also give the number in the unambiguous scientific notation (100×1018). 7 I haven't seen anyone suggest it, yet, but I would just say this as "One times ten to the twentieth [power]," where [power] is optional. To me, this seems less confusing when dealing with numbers of magnitudes above the trillions. It doesn't run into any of the regional interpretation problems listed in the answers above regarding the meaning of 'billion' and so forth and it doesn't sound (to me) as awkward as saying "hundred million million million." Incidentally, numbers are also commonly actually written that way. For example, scientists and engineers frequently write large numbers as 1.0 x 10^20 (or sometimes 1.0e20) rather than 100,000,000,000,000,000,000. No need to count the zeroes that way. This is commonly referred to as Scientific Notation. As a side note, you know your currency has inflation problems when scientific notation is useful for listing normal amounts of money. :) I can just imagine seeing scientific notation on paper money or on a store receipt or cash register. 1I guess you'd call that 'one-with-twenty-zeros', which is the most likely phrase to be understood. – Govert – 2015-06-25T18:39:08.767 5 # one hundred quintillion # a hundred quintillion This is the list of numbers (truncated): • ... • 1,000,000: a million • 1,000,000,000: a billion • 1,000,000,000,000: a trillion • 1,000,000,000,000,000: a quadrillion • 1,000,000,000,000,000,000: a quintillion • ... In the above list, you can replace "a" with a number from zero to nine. That means to multiply a number with another, e.g. • two hundred = a hundred * two = 100 * 2 = 200 • five billion = a billion * five = 1000000 * 5 = 5000000 "a" is the same as "one". Using the information above, here is the compilation of 100,000,000,000,000,000: 1. We see that the number has between 16 and 18 digits, so we're talking about A quintillion B. 2. We then see that the digits to the right are all zeroes, so we omit B, staying with A quintillion. 3. To find x, we look at the digits before the first digit separator. They are three digits, so we set A to C hundred D. 1. The first digit of them is 1, so we set C to either one or a, having a hundred D. 2. The other two digits are zeroes, omitting D (staying with a hundred). 4. Our final result is a hundred quintillion. 3 Different languages don't agree on the names of numbers 1,000,000,000 and larger. Therefore it's a good custom to express such large numbers using no bigger names than "million", since everyone in the world agrees on that. Otherwise non-native speakers might get confused, and not even all English speakers agree on the names of large numbers. Following that principle, 100,000,000,000,000,000,000 would be pronounced as "one hundred million million million". This would avoid any possible international confusion, even if it is a little long. 1It's especially bad when some text is translated from English say to German. If you read in German "eine Billion" you have absolutely no idea whether this is a correct translation of "one trillion" or an incorrect translation of "one billion", or a correct translation of "one billion" in an English text before 1960. And then you have no idea whether the original English writer knew what he was writing. – gnasher729 – 2016-06-11T19:08:16.150 3 You could also say 100 Exa Pengo or 100 EP, like some people (particularly in economics) say$100K to say 100 000 dollars. It's not very common, but the advantage is that there's no ambiguity compared to trillion/quintillion.

This is not a usage you will often hear informally, but it can pop up in newspapers and economics papers.

2No you couldn't. Just as you wouldn't say "ten kilodollars", "five megaeuro" or "nine gigapounds", you wouldn't say "one hundred exapengo". – David Richerby – 2015-06-23T18:47:11.947

3

@DavidRicherby Well, technically, you could. Indeed in the U.S., I occasionally hear "kilobucks" and "megabucks". These are playful usages, of course, not meant to be precise. But one always has the option of inventing a new usage to suit the occasion. It just wouldn't be standard (at that time), and people might laugh at you. ;)

– Ben Kovitz – 2015-06-23T19:12:35.630

1@BenKovitz I have also often heard the term kilo used when denoting money. Often the actual currency is left out if it is implied from the context that we are speaking about a sum of money. But I don't recall hearing higher SI prefixes used about money, and kilo I have only heard in informal speak. – kasperd – 2015-06-23T19:23:16.910

@BenKovitz I see no evidence that the question is asking for neologisms. Invented usage is almost never an answer on ELU. – David Richerby – 2015-06-23T19:28:06.957

5@kasperd I've heard "k" used ("He earns fifty k a year") but, in British English, I've never heard "kilo" on its own to mean anything other than a kilogram. – David Richerby – 2015-06-23T19:29:20.000

@DavidRicherby Good point. (I understood this answer as supplemental to some of the other answers, which complain about the million/billion ambiguity.) – Ben Kovitz – 2015-06-23T19:33:50.317

@DavidRicherby That is also my experience with American English. I did occasionally hear "kilo" used to mean "kilometer" or "kilometer per hour" while in the Philippines, though. It was only in situations where the unit was obvious from the context, though. – reirab – 2015-06-23T21:28:44.573

2

It's a bit hard to search for $...K but here is a concrete example in the news for$100k: $100k search on Google News - 646K results for me (pun intended). So yes, as I said, it's not very common, but it's used in newspapers and economics academic papers, so I thought it should be proposed as a complementary answer here. – gaborous – 2015-06-23T23:31:44.147 @gaborous But that's not a use of "kilodollars": it's a use of "k" as an abbreviation for one thousand. It would be read as "One hundred k dollars", not "one hundred kilodollars". In contrast, "100km" is "one hundred kilometers", not usually "one hundred k meters". – David Richerby – 2015-06-24T14:30:10.740 2Megadollar, MUSD and GUSD are used. SI prefix + ISO 4217 code is unambiguous. – Cees Timmerman – 2015-06-24T14:48:06.080 1 @DavidRicherby Are you serious? You're saying that k is an abbreviation for one thousand, but not for kilo? Maybe you're too used to hear it orally, but it is unambiguously referring to kilo. What else could it be referring to? And what would$...M dollars be referring to ? Is M not referring to Mega (or Million, this is equivalent), but to "x M dollars" or "x thousand thousand" ? And what about $...G ? etc. etc... – gaborous – 2015-06-24T17:09:07.810 @gaborous Yes, I'm being absolutely serious. We both agree that "$100k$means one hundred thousand dollars. I have never heard "$100k" read as "one hundred kilodollars". Nor have I ever seen it written as "$100kilo". Therefore, in my experience, that particular use of "k" does not stand for "kilo" or and is not referring to something called a "kilodollar". – David Richerby – 2015-06-24T17:22:33.760 1 @DavidRicherby I agree that the form "$..k" is not so common, and in fact I am surprised it can be used orally. But when written, I maintain that this is a direct abbreviation of kilo, just like "$...M" == mega dollars and "$...G" == giga dollars. The abbreviations are rarely expanded, because they are usually used exactly for the purpose of contracting a very long number onto a neat small but precise number, thus it would be awkward to develop the abbreviation, even if it's done sometimes, albeit, I agree, rarely.

– gaborous – 2015-06-24T21:52:52.363

2@DavidRicherby "k" means "×1000", but it stands for "kilo", regardless of how readers choose to speak it aloud. – Clement Cherlin – 2015-06-25T15:52:15.887

@ClementCherlin If it stands for "kilo", why is it never written as "kilo" and never referred to as a "kilodollar"? Its etymology is clearly through "kilo" but what does it mean to say that it stands for "kilo" if it's used in a way that "kilo" is never used? – David Richerby – 2015-06-25T16:15:25.087

@DavidRicherby It means that syntactically, "k" stands for "kilo", and semantically, "k" means "multiplied by 1000". Written notation and spoken language frequently differ. I write "$10" but I don't say "dollar ten" out loud. I may read "$0.50" as "fifty cents", "one half dollar", "zero point five dollars", "half of a dollar" and so on. Similarly, I may write "$10k" and say "ten thousand dollars", "ten kilodollars", "ten kay", "ten grand", "five stacks" or whatever. The way a given notation is read out loud does not affect its meaning. – Clement Cherlin – 2015-06-25T16:30:22.127 The discussion in this comments is dominated by programmers/scientists/engineers. "Normal" people would not talk about megadollars or gigadollars. – gerrit – 2015-06-26T17:24:25.137 1@gerrit no this is mainly used by economists, that's why it sometimes pops up in newspapers. It's not because you are used to often see the international system used in conjuction with bits/bytes unit that it's never used for other units... – gaborous – 2015-06-27T11:33:56.947 1@gerrit Indeed "normal" people seldom talk of huge numbers (except during hyperinflation). Happily for us all, the English language is not limited to the usages of "normal" people. :) – Ben Kovitz – 2015-06-27T12:22:50.870 3 As pointed out in several other answers and the comments on this answer, the words for very large numbers (billion, trillion, ...) have two different meanings depending on which variant of English you are speaking: long scale and short scale. Also, the words past "trillion" are rarely used and people may not remember what they mean. (Quick! How much is a vigintillion?) Therefore, I strongly recommend use of scientific notation to say numbers larger than 100 million (100,000,000) out loud, because it's unambiguous and it's easier for people to think about. Count the number of digits in the number, then say "a times ten to the Nth", where a is the first digit of the number, and N is the number of digits minus one. If a is 1 you can just say "ten to the Nth". For instance, the face value of that Hungarian note was ten to the twentieth pengo. If you need to, you can give more leading digits of the number by saying "a point b c" where a, b, and c are the first, second and third digits. Technically you can keep going with that as long as necessary, but I can't think of any situation where I would say more than three leading digits out loud. I might write 6.02214129 × 1023 on a chalkboard during a chemistry lesson, but I would still say "six point oh two times ten to the twenty-third." Especially when speaking of money, people often use an even shorter form: "N figures" means "a number which is at least 10N but less than 10N+1." For instance, "he makes six figures a year" means he makes at least$100,000 a year but not more than \$999,999 a year.

2Hmm... I'd say, at least in the U.S., most people will understand 'trillion' just fine, but I agree that saying "ten to the x" is better for numbers larger than the trillions. Trillions are used commonly when describing the size of the economy, the federal budget, the size of hard drives, or, for Obama's first few years, even the federal budget deficit. – reirab – 2015-06-24T15:52:42.650

1@reirab See kasperd's answer -- in casual conversation, confusion between a short-scale billion (10<sup>9</sup>) and a long-scale billion (10<sup>12</sup>) probably isn't going to be a huge deal, but when you hit trillions (10<sup>12</sup> short, 10<sup>18</sup> long) I don't feel safe with the ambiguity anymore. – zwol – 2015-06-24T17:06:25.753

2I've actually never seen trillion used to mean 10^18 in the U.S., though I'm aware that it's used for that elsewhere. Here, though, trillion is widely understood to mean 10^12 and it's used frequently enough that I think most people understand it without any problem. – reirab – 2015-06-24T18:08:02.800

@MyStream The trouble comes when you impose your own definition of "count correctly" that doesn't actually match usage in English (to my knowledge, in no English-speaking country does "billion" refer to 10^12 these days). Please stop thinking about etymologies when considering what is correct; they are irrelevant. What is relevant is "what will make you understood, and what will leave people thinking you're saying something other than what you're actually saying;" in English, using "billion" to mean 10^12 is normally the latter. – cpast – 2015-06-25T06:27:18.940

There is no right or wrong here. Both the long and the short scale are "the way counting is done" in some but not all Englishes. It is possible to cause serious confusion if you speak in one scale but your interlocutor expects the other, and that is why I suggest the use of ten-to-the-Nth for numbers larger than 10^11. Maybe I should revise that down to 10^8. – zwol – 2015-06-25T13:04:56.550

@MyStream I certainly agree that history is always essential to understanding the meaning of a word, and etymology educates. In this case, I think the relatively recent precedent for the short scale has made the long scale mostly obsolete. But people can reasonably and respectfully disagree about this. Would you be willing to write an answer in terms of the long scale, targeted at ESL learners and explaining the history? It might be a useful addition to the other answers. At the very least, it will illustrate how English is a language filled with competing precedents and controversies. – Ben Kovitz – 2015-06-25T14:58:44.690

@MyStream BTW, as I understand the long scale, a trillion is not a billion billion, but a million million million (a million "thrice"). If I'm mistaken about that—well, all the more reason to have a thorough answer in terms of the long scale. – Ben Kovitz – 2015-06-25T15:02:55.493

@MyStream To reiterate: You are claiming that long scale counting is the "correct" scheme. This is wrong in English in general. Someone who uses the long scale system in most English-speaking countries is counting incorrectly. It is not "simply the way counting is done;" again, in most English-speaking countries, long scale counting is incorrect. Short scale counting is not an error, it is how English works in most English-speaking places. – cpast – 2015-06-25T19:00:58.917

@MyStream Your reputation is high enough to post an answer. – Ben Kovitz – 2015-06-26T15:10:33.950