16 C.F.R. § 500.23
(b) The following symbols for SI metric units and none others may be employed in the required net quantity declaration:
centimeter—cm cubic centimeter—cm 3 cubic decimeter—dm 3 meter—m milligram—mg liter—L or l milliliter—mL or ml square decimeter—dm 2 cubic meter—m 3 kilogram—kg micrometer—µm gram—g millimeter—mm square meter—m 2 square centimeter—cm 2
Note: Symbols, except for liter, are not capitalized. Periods should not be used after the symbol. Symbols are always written in the singular form.