In numbers more than four digits long, use commas to separate the numbers into groups of three, starting from the right. In numbers four digits long, a comma is optional.
3,500 [or 3500]
100,000
5,000,000
Do not use commas in street numbers, zip codes, telephone numbers, or years.
Exercises:
Major uses of the comma 1
Major uses of the comma 2
All uses of the comma
Related topics:
Spelling out numbers
Acceptable uses of numerals