Rules for using Arabic numerals or spelling out numbers are as follows and are listed from highest priority to lowest.

If the number is part of a casual expression, spell it out.

If the number is a calendar year, do not spell it out.

If the number is an age or percentage, do not spell it out.

If the number begins a sentence, spell it out. Awkward sentences should be reformed.

If the number is greater than 10, do not spell it out.

If the number is one through nine, spell it out.

Use Roman numerals for wars and honorific suffixes.

World War II

John Doe III

Cardinal numbers include figures 1, 2, 10, 101, and so on, and the corresponding words.

Ordinal numbers include the terms 1st, 2nd, 10th, 101st, and so on, and the corresponding words.

When spelling out large numbers, connect words ending in 'y' to subsequent words within the same number with a hyphen. Avoid commas between words that are part of one number.

twenty-one

one hundred thirty-one

twenty-five thousand one hundred thirty-one

ninety bottles

Write proper names according to the owner's practice.