NGS Style Manual‎ > ‎- K -‎ > ‎

kilometer, km

Spell out in text. Abbreviate km in map notes, no period:
            32-km trip.

In editorial copy kilometers per hour may be abbreviated kph, though it is best to spell it out.
(Note that the official international system for metric uses km/h.)

See also centimeter, cm, meter, mmillimeter, mm, METRIC AND INTERNATIONAL MEASUREMENT.