When should I use a comma to separate numbers?

In English, we use commas to separate numbers greater than 999. We use a comma every third digit from the right.

• More than 50,000 people turned up to protest.

The comma every third digit is sometimes known as a “thousands-separator.” Make sure you don’t include a space on either side of this comma.

Correct:

• We will walk 10,000 miles.

Incorrect:

• We will walk 10000 miles.
• We will walk 10, 000 miles.
• We will walk 10 , 000 miles.
• We will walk 10 ,000 miles.

When a number uses a decimal point, we never place commas to the right of the decimal point. Some people like to use thin spaces going from left to right instead.

Correct:

• The value of Pi is 3.14159 to five decimal places.
• The value of Pi is 3.14 159 to five decimal places.

Incorrect:

• The value of Pi is 3.141,59 to five decimal places.
• The value of Pi is 3.14,159 to five decimal places.

Numbers that are not amounts, such as phone numbers, house numbers, and years do not usually have commas.

It can get confusing as many other countries (such as Spain) use commas in numbers instead of a decimal point, e.g. 100,01 instead of 100.01. In these cases, you might see a period used instead of commas or a space, e.g. 1.000,01 or 1 000,01 instead of 1,000.01. But for English-speaking countries, follow the rules above.