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Using Commas with Numbers

The Quick Answer
Commas can be used with large numbers to assist the reader. They are placed every three decimal places to the left of the decimal point. For example:
  • 123,456.789
Be aware that many other countries use a comma for the decimal point and full stops / periods to mark out every three decimal places. For example:
  • 123.456,789
  • (This would be correct in many non-English-speaking countries, such as Germany and France.)

Using Commas with Numbers

Commas can be used to make reading numbers a little easier. They are placed every 3 decimal places for numbers larger than 999.

Examples:
  • 1,234
  • 23,566
  • 1,234,967
  • 8,000
  • 8,000.01
  • 8,000.0123456
  • (Commas are not used to the right of the decimal point.)

Do Not Add Commas to Serial or Reference Numbers

Do not put your own commas in serial numbers, reference numbers, or 4-figure years. For example:
  • According to the document with reference number 912345, serial number 1654880 was destroyed in 1984.
When years are not in the 4-figure format, it is possible to use commas. For example:
  • The film is set in 10,000 BC.
Note

No Commas to the Right of the Decimal Point

Do not use commas to the right of the decimal point.
  • 4,567.123456
  • 4,567.123,456
  • (This would be considered unusual.)
Beware

Beware Other Conventions for Writing Numbers

The use of commas and full stops in numbers is not consistent across the world. Below are the four most common conventions:
  • 4,567.1
  • (United Kingdom, United States, China)
  • 4.567,1
  • (This convention is used in most of Europe. It is the convention used by most countries.)
  • 4 567,1 and 4 567.1
  • (The practice of using a space to separate the 1000s is also common.)