Learn English – Should Units of Measure be capitalized?

No, units generally do not need capitalization when spelled out. For SI units, the Bureau International des Poids et Mesures is the authority:

Unit names are normally printed in roman (upright) type, and they are treated like ordinary nouns. In English, the names of units start with a lower-case letter (even when the symbol for the unit begins with a capital letter), except at the beginning of a sentence or in capitalized material such as a title. In keeping with this rule, the correct spelling of the name of the unit with the symbol °C is "degree Celsius" (the unit degree begins with a lower-case d and the modifier Celsius begins with an upper-case C because it is a proper name).

                                    — The International System of Units (SI), 8th ed., Sec. 5.2

However, capitalization is used in the following cases:

• Symbols for SI units named after people, for example:

  • 1 ampere, abbreviated as 1 A
  • 1 angstrom or 1 ångström, abbreviated as 1 Å

  Note that the names of the units themselves are lowercase.

• The symbol for litre or liter is l, but is often written as L to prevent it from looking like the digit 1. It is also sometimes rendered in lowercase cursive form ℓ for the same reason:

  The litre, and the symbol lower-case l, were adopted by the CIPM in 1879 (PV, 1879, 41). The alternative symbol, capital L, was adopted by the 16th CGPM (1979, Resolution 6; CR, 101 and Metrologia, 1980, 16, 56-57) in order to avoid the risk of confusion between the letter l (el) and the numeral 1 (one).

                                      — The International System of Units (SI), 8th ed., Sec. 4.1

• Temperature degree units, e.g. degrees Celsius or degrees Fahrenheit, abbreviated as °C and °F, as stated in the quote above.

  Note, however, that the kelvin follows the rule for units named after people: 1 kelvin, abbreviated as 1 K.

• Special non-SI units, such as the British thermal unit, abbreviated BTU or Btu, or the US gallon, for obvious reasons.

• The astronomical unit is abbreviated either as au or AU. The international unit is abbreviated IU.

• A byte is abbreviated as B; a bit is abbreviated as b.

• A decibel is abbreviated as dB.

One unit where capitalization makes a huge difference is the calorie.

• A calorie (abbreviated cal) is the energy needed to heat one gram of water by 1 °C. This unit is used by physicists and chemists.

• A Calorie (abbreviated Cal or kcal) is actually a kilocalorie — 1000 times the amount of energy of a calorie. This unit is used by dieticians and the general public when describing food.

Naturally, at the beginning of a sentence, the usual rule that the first letter be capitalized overrides all of the above. For example,

Watt for watt, a RISC chip delivers better performance than a CISC chip.

Also note that capitalization is significant in the SI prefixes. For example, mW is a milliwatt, and MW is a megawatt.