Rules for writers, Tenth edition - Diana Hacker, Nancy Sommers 2021

Abbreviate units of measurement used with numerals
Abbreviations
Mechanics

Generally, use the abbreviations for units when they appear with numerals; spell out the units when they are used alone or when they are used with spelled-out numbers (see also 42a).

Results were measured in pounds.

Runners in the 5-km race had to contend with pouring rain.

Use no periods after abbreviations for units of measurement. Only the abbreviation for “inch” (in.) takes a period, to distinguish it from the preposition in.