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Units of Measure

Phil B. Fontanarosa

and Stacy Christiansen

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The SI unit names are written lowercase (eg, kilogram) when spelled out, except for Celsius (as in “degrees Celsius”), which is capitalized. Abbreviations or symbols for SI units also are written lowercase, with the following exceptions:

  • Abbreviations derived from a proper name should be capitalized (eg, N for newton, K for kelvin, A for ampere), although nonabbreviated SI unit names derived from a proper name are not capitalized (eg, newtons, amperes).

  • An uppercase letter L is used as the abbreviation for liter to avoid confusion with the lowercase letter l and the number 1.

  • Certain SI prefixes are capitalized to distinguish them from similar lowercase abbreviations:

    • M denotes the prefix mega (106), whereas m denotes the prefix milli (10−3)

    • mg denotes milligram (10−3 g), whereas MHz denotes megahertz (106 Hz)

    • P denotes the prefix peta (1015), whereas p denotes the prefix pico (10−12)

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