How to Write Numbers in English
Here are some easy rules to help you along:
1. In English, we write numbers starting with the largest digit
This is just the basics, but when we're writing or saying numbers, we start with the highest value digit first. So 21 is written as "twenty-one" (not "one-and-twenty" as it is done in many languages).
2. Write single numbers out.
Use digits (the numbers from 0 - 9) for longer numbers. So for "8" you would write "eight", but for "213" just write the numbers out. If you need to use two words to write a number, then it's probably better to just write out the digits.
3. The Decimal Comma
In English, we use a comma as a decimal space, rather than a period (full stop) or a space. So one thousand five hundred is written as 1,500. We use a period to indicate a fraction of a number. For example, one and-a-half would be written as 1.5.
4. Don't start sentences with a number
It just makes using capital letters difficult. So instead of saying "400 tickets were sold" we would say, "Supporters bought 400 tickets."
5. Dates should be numbers, but centuries and decades should be written out
So we would write "1980", but talk about "the eighties" or "the twentieth century".
6. For really big numbers, a combination of digits and words is okay
So 100,000,000 would normally be written as "100 million" - it's easiest to read and understand.
7. Decide how you will write numbers, and stick to it
It's usually up to you how you want to write numbers, but the important thing is to be consistent. So if you start writing them out in words, then you need to stick to that for the rest of your letter or essay.