Having gone to school back in the mid 80's to the early 90's, I pretty much sucked at math and English. However, one of the things I remembered was that the first comma in a large number was not inserted unless the number had at least 5 digits.

Example -






However, I never really tried looking that up until now, when it came up in discussion. I can't seem to find a reference to this 'rule' or guidance anymore, and am wondering if - in my old age - I'm just completely making that up now.

  • 1
    I've seen plenty of instances of, e.g. "$1,000". I've never heard of this "Wait til the fifth digit" rule.
    – Dan Bron
    Commented Dec 8, 2016 at 17:27
  • 4
    It's an SI rule, but SI uses spaces as thousand-separators.
    – Andrew Leach
    Commented Dec 8, 2016 at 17:29
  • What research did you do on your own before asking here? If you cite where you've looked, other people won't look the same places. Commented Dec 8, 2016 at 17:53
  • 1
    Interesting, thanks guys. I specifically remember it being a comma and not a space, and NOT for currency (@danBron's comment) - just specific text like "Today, 45,522 people celebrated their birthday, and tomorrow only 4234 people will." The two main sites I checked were the AMA and Rules for Writing Numbers. I did come across the SI site as well, and saw that it had referenced spaces. I'm guessing that sometime between learning in school, and now, I got the two merged in my head, thinking they were one and the same.
    – Coyttl
    Commented Dec 8, 2016 at 18:27
  • 3
    It's purely a matter of "style". In documents of a scientific or financial nature it's best to insert separators beginning at 1,000, to avoid confusion, but in less rigorous circumstances the comma is often omitted to reduce clutter.
    – Hot Licks
    Commented Dec 8, 2016 at 19:33

2 Answers 2


This is a question of style, and, if you are writing in a structured environment (e.g., school or work), you should follow whatever standards and guidelines are in effect in your situation.  But, as one somewhat official example, the U.S. Government Printing Office (GPO) Style Manual (a 467-page PDF) says, in Chapter 12. Numerals, section 12.14 Punctuation (page 289) the following:

The comma is used in a number containing four or more digits, except in serial numbers, common and decimal fractions, astronomical and military time, and kilocycles and meters of not more than four figures pertaining to radio.

although in preceding paragraphs they have indicated that commas should not be used in years (e.g., 2017).

Section 12.7 indicates that street addresses (e.g., “1727 St. Clair Avenue”) and telephone numbers are considered to be serial numbers (and thus should not have commas — presumably, even if they have more than four digits (and, yes, I know of a place where they have five-digit street numbers)).  It goes without saying that this also applies to postal ZIP codes.

Oddly, they say that “a 1,100-percent increase” and “an 1100-percent increase” are both acceptable.  They don’t explain; I guess the rationale is that “1,100” would be pronounced “one thousand one hundred” and “1100” would be pronounced “eleven hundred”.

And (again, without any explanation) they offer “Dow Jones average of 10500.76” as an example of correct usage.

  • Gonna go ahead and mark the answer. My research didn't find the GPO links before, that's good to see. +1 for the difference in readings of '1100-percent', too..!
    – Coyttl
    Commented Dec 11, 2016 at 20:49

I've seen this "rule" before. In my opinion, it is a stupid rule.

"2017 graduates": is 2017 a year or a head count? Without that "rule", you could write:
"2017 graduates" (2017 is a year) or
"2,017 graduates" (2,017 is a head count)

Also, in tables, the numbers will not line up right if you include commas in some numbers ≥ 1000 but not others.



  • 2
    Strange rule. As much as I know there are no rules how a delimiter must be used. Commented Dec 9, 2016 at 6:14

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