To write numbers correctly, spell out small numbers, centuries and decades and all numbers beginning a sentence. Small numbers include whole numbers less than ten. For clarity, use noon and midnight instead of 12:00 p.m. and 12:00 a.m., respectively.
Except for a few basic rules, spelling out numbers or using figures largely depends on the writer's preference. However, consistency is the key. Basic rules govern writing of small numbers, percentages, ordinal and compound numbers.
Hyphenate all written-out fractions and compound numbers from twenty-one through to ninety-nine. Do not hyphenate number terms such as a third, a quarter or a half. To make figures of four or more digits easy to read, use the comma separator. Count three spaces to the left of the digit and place the first comma. Continue placing commas after every three digits. For decimal numbers, this rule only applies to the numbers on the left of the decimal point. When using percentages in everyday writing and recipes, write them in digits but in formal writing, spell the percentage out.
To write two numbers next to each other, make the number that has the fewest letters a numeral. Spell out all rounded and estimated numbers. Rounded numbers which are over a million should appear as a numeral and a word, for instance, 100 million. Express mixed fractions in figures unless they begin a sentence.