The rules on when to use an actual number and when to spell a number in professional writing can be a little confusing. To me, they can also seem a bit arbitrary. Each company, media outlet and organization tend to have rules for writing numbers when sending out official information, but I often encounter mistakes in instances such as when to write “5” or “five.” Here are a few simple rules.

• If the number is less than 10, spell it out. If greater than 10, use the number. For example, one hundred would be 100 and five would be five in whatever you are writing. Read on for the exceptions.
• When the number is at the beginning of a sentence, always spell it out.
• When mixing large and small numbers in one sentence, use the number. For example, there are 5 horses and 10 chicken on the farm.
• When referring to age, percentages or other specific numbers, always use the number. For example, 5 years old, 2 percent page 7.
• When talking about very large numbers, use this number and word combination: 2 million, 16 trillion, etc.
• Always use a number when talking about a specific date and time. For example, 2 o’clock, 6:30 p.m., Oct. 7
• When referring to ordinal numbers, such as first second third, follow the same general rule as regular numbers. For example, first, fourth, ninth, 10th, 15th.

Here’s a basic, but easy to read reference beyond the one provided above, but if you want more a more in-depth look at how to use numbers in your writing, this article gives good advice. Of course if you do not want to follow the APA or AP Stylebook rules regarding numbers, there is always the finicky Chicago Manual of Style, which is explained well here.