units of measurement

Use numerals with units of measurement and time in technical papers and reports, even when the number is less than 10. In some outreach publications, you may spell out numbers less than 10, especially with units of time.

Except with $, °, and %, leave a space between the numeral and the unit. Include a hyphen between a numeral and an abbreviated unit when the value is used as an adjective.

Do not include a period after a unit symbol except at the end of a sentence. The exception to this rule is inches (which are abbreviated “in.”).

  2 kW 7 cm2 16.8%
  3 m 8-hour days 300 Btu
  5 years $2 billion 45°
  4 in. a 15-MW turbine 65°C

Avoid nonstandard abbreviations such as sec or cc, and instead use standard abbreviations such as s and cm3. Unless your profession, technical field, or scientific discipline specifies something different, use the abbreviations in NREL's list of technical abbreviations or the National Institute of Standards and Technology (NIST) guide.

Use a center dot to indicate multiplication of units (such as N∙m for Newton-meters), and use a slash or negative exponent to indicate division. For example, 344 meters per second can be written as 344 m·s−1 or 344 m/s.

Format variables in italic type and units in roman type:

Correct: t = 3 s, where t is time and s is seconds
Incorrect: t = 3 s, where t is time and s is seconds
Do not mix information with unit symbols or names:

Correct: the water content is 20 mL/kg
Incorrect: 20 mL of water/kg
Avoid mixing symbols and names; for example, use kg/m instead of kilogram/m.

When expressing a range, repeat the unit symbol if it is closed-up with the numeral ($, °, %):

60°–90° (example of angular measurements)

Refer to the NIST style conventions checklist for additional style tips and examples.