Eight Rules for When to Spell Out Numbers

Numbers don't just show up in mathematics, they also appear in everyday writing. Like many facets of the English language, there are rules for writing numbers. Certain numbers are spelled out with letters and others are only written as numerals.

1. Spell out numbers under 10

When writing numbers under 10 in a sentence, they should be spelled out. This is true regardless of where they fall in a sentence.

Mary read four new books last week and seven newspaper articles.

2. Use numerals for numbers 10 and above

When you include numbers 10 or higher in sentences, those should be expressed as numerals, unless they are at the very beginning of the sentence.

I am planning to take a Caribbean cruise with 10 of my closest friends.

She's bought about 12 pairs of shoes and 16 dresses in the last three months.

3. Spell numbers that begin sentences

Whenever there are numbers at the beginning of a sentence, those numbers should be written out.

Sixty children came to the class trip last year but, this year, there were 80.

4. Use numerals in headings and titles

When you include a number in a heading or a title, it's better to use a numeral rather than spelling out a word.

• That's why you'll see ‘Top 10’ in the title of list articles rather than ‘Top Ten’. This rule is true even when the number is the first word of the title
• Title an article ‘15 Best Restaurants in London’ rather than spelling out fifteen
• For a title or heading within an article, write ‘Top 6 Must-Remember Rules’ rather than spelling out six

5. Hyphenate spelled-out compound numbers

When compound numbers are spelled out, such as when they are at the beginning of a sentence, they should always be hyphenated.

Fifty-two miles were all she had left on her journey to Scotland.

6. Use numbers for percentages

When writing percentages, use numbers paired with the % sign.

• According to the latest survey, 52% of teachers live in the city
• It's good to know that only 7% of Americans say they are unhappy

The word ‘percent’ should generally only be used when spelling out percentages in words, which you would only do at the start of a sentence.

7. Avoid mixing number words and numerals in lists

When numbers are in a list, it is generally ideal to keep all the numbers in the list consistent, even if some numbers are under 10 and some are over.

• Inconsistent: she has four brothers aged seven, nine, 12, and 15
• Consistent: she has four brothers aged 7, 9, 12, and 15
• Inconsistent: Mary's travelled to three European countries and 14 deserted islands
• Consistent: Mary's travelled to 3 European countries and 14 deserted islands

8. Vary format with two side-by-side numbers

When you're writing a sentence that has two numbers side-by-side that aren't part of a related list, you'll need to adjust the text for clarity. In this case, it is best to write one number as a word and the other as a numeral.

• Best: there are going to be 12 ten-year-old kids at my house this weekend
• Confusing: there are going to be 12 10-year-old kids at my house this weekend
• Less confusing, but not ideal: there are going to be 12, 10-year-old kids at my house this weekend

Another option would be to rearrange the sentence to separate the numbers. You could say, ‘there will be 12 kids who are 10 years old at my house this weekend’.

Sources

• https://www.dailywritingtips.com/10-rules-for-writing-numbers-and-numerals/
• https://grammar.yourdictionary.com/grammar-rules-and-tips/rules-for-writing-numbers.html
• https://www.scribendi.com/academy/articles/when_to_spell_out_numbers_in_writing.en.html
• https://www.grammarly.com/blog/when-to-spell-out-numbers/
• https://www.grammarbook.com/numbers/numbers.asp