Students often ask me about how to deal with writing numbers. A messy subject, because except for a few basic rules, spelling out numbers vs. using figures (also called numerals) is largely a matter of writers’ preference (and a choice between British English and American English). Again, consistency is the key. Some firms might even have a Company guidebook or stylebook with a pre-described house style. It might be useful to check these. Whatever you use, be consistent!!
1. Spell small numbers out. The small numbers, such as whole numbers smaller than ten, should be spelled out. That’s one rule you can count on. Not spelling out numbers is acceptable in an instant text message, but not suitable for a formal document.
2. No other standard rule. Experts sometimes disagree on other rules. Some say that any one-word numbers should be written out. Two-word numbers should be expressed in figures. That is, they say you should write out twelve, sixteen or twenty, but not 24. (An exception is made for years e.g. “1968 was a tumultuous year”). Whatever your choice is: be consistent within the same sentence! If I have 23 beginning students, I also have 18 advanced students, not eighteen advanced students.
3. Hyphenate all compound numbers from twenty-one to ninety-nine. You should write 42 as forty-two,. 832 should be written as eight hundred thirty-two and 2361 as twenty-three hundred sixty-one. Moreover, fractions used to be written with hyphens e.g. three-quarters, but you will see them without nowadays e.g. three quarters. Don’t use a hyphen between a whole number and a fraction e.g. two and three quarters OR two and three-quarters.
4. If the number is rounded or estimated, spell it out. Rounded numbers over a million are written as a numeral plus a word. Use “About 400 million people speak Spanish natively,” instead of “About 400,000,000 people speak Spanish natively.” If you’re using the exact number, you’d write it out, of course.
5. Using the comma. This is another grey area. In British English, the comma is used as a thousands separator (and the period as a decimal separator), to make large numbers easier to read. So write the size of Alaska as 571,951 square miles instead of 571951 square miles. In Continental Europe the opposite is true, full stops are used to separate large numbers and the comma is used for decimals. Finally, the International Systems of Units (SI) recommends that a space should be used to separate groups of three digits, and both the comma and the full stop should be used only to denote decimals, e.g. $13 200,50.
6. Percentages. Percentages can be either written out e.g eighty per cent OR written in figures e.g. 80%. Remember, consistency is key!
7. Two numbers next to each other. It can be confusing if you write “7 13-year-olds”, so write one of them as a numeral, like “seven 13-year-olds”. Pick the number that has the fewest letters.
8. Expressing decades. Again: widespread confusion… Most British writers prefer to spell out centuries and decades as: the Eighties or the nineteenth century. American writers more often than not put an apostrophe before the incomplete numeral and no apostrophe between the number and the s e.g. During the ’80s and ’90s, the U.S. economy grew. When spelling out decades, Americans do not capitalize them, the British do.
9. Try not to start a sentence with a numeral. Make it “Eight hundred years ago,” not “800 years ago”. This means you might have to rewrite some sentences: “Fans bought 400,000 copies the first day”instead of “400,000 copies were sold the first day”, thus eliminating the dreaded passive voice. The exception to this rule is calendar years i.e. you can write ‘1984 was the start of a difficult year in British politics’ (see also under point 2, above).
10. Dates are written in all-figure form e.g. 22/08/2014. However, remember that there is an important difference between British/European style and American style. In the US, they write the month first and then the day i.e. 08/22/2014. Moreover, UK English uses cardinal numbers e.g. 22 August 2014 rather than ordinal numbers e.g. 22nd August 2014.