When to spell out numbers depends in good part on what kind of writing you’re doing.
In books for general readerships, the standard is The Chicago Manual of Style’s “general rule” of writing out whole numbers from zero to one hundred.
In newspaper journalism, whole numbers from zero to nine are written. After nine, numbers are expressed in arabic numerals: 10, 11, 12, etc.
There are exceptions to the above rules. Chicago, for example, has an “alternate rule” of “spelling out only single-digit numbers” and suggests that it sometimes could make sense to stick to digits in a series of measurements that are both below and above 100.
The important thing is to be consistent.
Chicago and the stylebooks for The Canadian Press and The Associated Press all offer detailed guidance on how to express numbers in print.
The CP Stylebook directs reporters to “write increased to 15 per cent from 10 (not increased from 10 to 15 per cent),” because increased from 10 to 15 per cent could be read to mean there was an increase of 15 per cent at most.
The Canadian news agency says a number should be spelled out at the start of a sentence, with a notable exception for calendar years: 2025 began quietly in Kipling, Sask. But you should probably recast the sentence, if you can, to avoid beginning with the year.
The AP Stylebook has guidelines for numbers that are similar but not identical to CP’s. It advises spelling out one through nine – “in general.” One exception is with ages: AP says to write The law is 8 years old. In CP style, you would spell out eight.
CP advises, by the way, that “writers should mind their math” when presenting statistics. I think that’s important advice.
Know the difference between a “Y per cent drop” and a drop of that many percentage points. As CP puts it: “A poll that suggests a drop in government support to 25 from 50 per cent does not mean a 25 per cent drop. It’s a change of 25 percentage points.”
Also, understand that a doubling is a 100 per cent increase, a tripling is a 200 (not 300) per cent increase, a quadrupling is a 300 per cent increase and so on. And “three times as much” is not the same as “three times more” (which is four times as much).