Spell out numbers less than ten in body copy. Ten and above get numerals, unless they’re the first word in a sentence.
Ordinal numbers (e.g. “first,” “31st”) follow the same rule.
However, always use numerals when writing:
- Sums of money
- Time of day
In headings, titles, calls to action, and other very small spaces, use numerals if it saves space or has more of a visual impact.