Official US writing rules for metric symbols

A benefit of the International System of Units (SI) is that written technical information is effectively communicated, overcoming the variations of language, including spelling and pronunciation. Values of quantities are expressed using Arabic symbols for numbers paired with a unit symbol, often with a prefix symbol that modifies unit magnitude. International SI style and usage publications focus on written communication. Verbal pronunciation of SI terminology is purposefully not addressed in the BIPM SI Brochure (French is the primary language, English is secondary), NIST SP 330 (uses American English spelling), NIST SP 811 (uses American English spelling), or IEEE/ASTM SI 10.

NIST SP 811 provides an editorial checklist for reviewing manuscripts' conformity with the SI and the basic principles of physical quantities and units.

This is the complete section on the NIST website for all things metric: https://www.nist.gov/metric