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I know that it is common to write the number 'ten thousand' as a numeral with a comma delimiting the 10 and the 000 like so: 10, 000

However, I have never seen a comma used for numbers less than 10,000, such as 8000 where, I assume using a comma would look like: 8,000.

Are there any rules relating to when a comma should be inserted into a numeral?

N.b. I live in the UK, so answers pertaining to UK English are preferred.

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It's just preference. The only reason larger numbers have them are because there are usually too many zeroes to eye. Numbers in the low thousands you can easily tell how many zeros there are without commas. –  ryan Nov 17 '13 at 12:51
In Indian English, 1 lakh is written out as 1,00,000 while 1 crore (100 lakh) is shown as 1,00,00,000 and a lakh crore of rupees or Rs 10,00,00,00,00,000 may be shown as Rs 1 lakh cr, which is useful to know if you are a big spender. ¶ “In numerical expressions German uses a comma where English uses a decimal point: €19,95 (19.95 euros) In large numbers, German uses either a space or a decimal point to divide thousands: 8 540 000 or 8.540.000 = 8,540,000” – Item 3.D. at a German Punctuation page –  jwpat7 Nov 17 '13 at 17:01
I was taught in math class, when I was a child, that you don't use the comma for 4 digit numbers, only 5 and up. –  user81811 Jun 21 '14 at 12:14

2 Answers 2

up vote 8 down vote accepted

In the English-speaking world, it is common to use commas every three decimal places in numbers of four or more digits, counting right to left.

When you do use a comma for a thousands-separator, do please make sure to write the digits flush against the comma — that is, without a space to either side of the comma: 10,000, unlike in your own example.

The only common exception to this common practice of using commas for a thousands separator is in 4-digit years, like “in 1776 ᴀᴅ” or “around 1500 ʙᴄ”. But if you are using more digits, even years take them, as in “around 15,000 ʙᴘ” (before present). The lack of a comma in a 4-digit figure is one way of indicating that it is meant to be read out in hundreds not thousands; for example, “around fifteen hundred”.

This makes them easier to read, and is even more important in a font like Georgia with proportional-width digits (the font you are presumably currently reading this posting in):

  • The constant c is 299792458 => 299,792,458 (meters/second)
  • The constant c is 670616626.969746072 => 670,616,626.969746072 (miles/hour)
  • The constant c is 1802617493294.67744153600 => 1,802,617,493,294.677441536 (furlongs/fortnight)

You don’t ever see commas to the right of the decimal point, but you might see thin spaces, this time going left to right instead:

  • The constant c is 299792458 => 299,792,458 (meters/second)
  • The constant c is 670616626.969746072 => 670,616,626.969 746 072 (miles/hour)
  • The constant c is 1802617493294.67744153600 => 1,802,617,493,294.677 441 536 (furlongs/fortnight)

And indeed some people, including Bringhurst himself (albeit this depends on whether you’re using titling figures or not), recommend the use of thin spaces instead of commas:

  • The constant c is 299792458 => 299 792 458 (meters/second)
  • The constant c is 670616626.969746072 => 670 616 626.969 746 072 (miles/hour)
  • The constant c is 1802617493294.677441536 => 1 802 617 493 294.677 441 536 (furlongs/fortnight)

In computer programming, you really cannot get by with either a comma or a thin space. Coding typically uses an underscore to represent a space (like between words in identifiers), and some languages such as Perl and Java indeed permit the underscore in numbers:

  • The constant c is 299792458 => 299_792_458 (meters/second)
  • The constant c is 670616626.969746072 => 670_616_626.969_746_072 (miles/hour)
  • The constant c is 1802617493294.677441536 => 1_802_617_493_294.677_441_536 (furlongs/fortnight)

See how much easier those are to read?

Of course, you can also use scientific notation, such as Avogadro’s famous number of 6.02 (well, 6.022 141 29) × 10²³ in “scientific” notation, or 6.02e23 in programmer notation.

That would lead to a figure of 1.802 617 × 10¹² or 1.802617e+12 for the last figure, which helps in some regards and hurts in others. Heck, unless high precision is needed, just say ~1.8 trillion and be done with it.

See also this question.

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Using spaces has the small advantage that (AFAIK and hopefully) no country uses it as decimal mark, while the comma is widely used in some countries (including Italy) instead of the dot. According to wikipedia the comma is the ISO standard as decimal mark, and the 22nd General Conference on Weights and Measures declared in 2003 that "the symbol for the decimal marker shall be either the point on the line or the comma on the line". It further reaffirmed that " neither dots nor commas are ever inserted in the spaces between groups –  Bakuriu Nov 17 '13 at 16:44
It's quite common only to use comma separators for numbers 10,000 or larger, which is the OP's question. But using commas for 1000 and larger (except years) is not considered bad style. However, you should be consistent. –  Peter Shor Jun 21 '14 at 14:26

I have no definitive authority on this but believe that commas should be inserted in any number which runs into the 000s and not just after 10,000.

Ergo; the example 8,000 should also have a comma.

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There is another convention where a gap is left rather than a comma inserted, usually for 10 000 and above. As a former science and maths teacher, I've never come across an instruction in this area claiming to be a rule rather than a sensible recommendation. –  Edwin Ashworth Nov 17 '13 at 15:13
I switched to the "new" convention Edwin Ashworth mentions about 10 years ago when I began teaching. It is cleaner in appearance than the comma-grouped convention. Does anybody remember if VisiCalc allowed the use of commas in numbers? If not, that could have been a justification for the comma-free approach. –  Michael Owen Sartin Nov 17 '13 at 20:12
@Michael Owen Sartin Justification? Explanation perhaps ('Ben Rosen speculated in July 1979 that "VisiCalc could someday become the software tail that wags (and sells) the personal computer dog". He was correct') (Although Antic wrote, "VisiCalc isn't as easy to use as prepackaged home accounting programs, because you're required to design both the layout and the formulas used by the program.). Apparently, VisiCalc did not require any separators at all. –  Edwin Ashworth Nov 18 '13 at 11:45

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