The Editor’s Manual
Free learning resource on English grammar, punctuation, usage, and style.
Abbreviated SI and other metric units do not have a separate plural form: don’t add “s” to form the plural of an abbreviated unit of measurement (10 km, not 10 kms).
Spell out single-digit numbers from zero to nine and larger numbers at the start of a sentence. Use numerals with abbreviated units of measurement, to emphasize the exact time, and to write the names of years.