In numbers of more than three digits, use a comma after every third digit from right to left.
Incorrect: The area of North America is approximately
9435000 square miles.
Correct: The area of North America is approximately
9,435,000 square miles.
(This is much easier to read.)
Numbers which normally do not take commas are ZIP codes, phone numbers, page numbers, serial numbers, house numbers, and dates of years.
Many European countries use a comma in place of the decimal point and use periods or blank spaces to separate every third digit.
United States: 2,367.48 francs
France: 2.367,48 francs or 2 367,48 francs