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I just created an inch to mm conversion calculator and I am writing a blog post announcing its launch. When should I capitalize the terms inch and millimeter? Does it matter if I'm spelling it out? What if I'm using an abbreviation or symbol? What if it begins a sentence?

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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:

    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 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.

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    You are wrong that the symbol for litre is L. It's a lower-case l -- not that that stops people from using L, but a blanket statement is inappropriate. Perhaps it's a North American thing; L rarely appears on the European side of the Atlantic. – Andrew Leach May 29 '14 at 21:27
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    @AndrewLeach I've added a citation. – 200_success May 29 '14 at 21:35
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    The alternative L still isn't common in Europe after 35 years. I maintain that what could be interpreted as a blanket statement is wrong though -- is abbreviated to should be can be shown by. [Note too that SI units have symbols, not abbreviations. BTU is not an SI unit and is an abbreviation] – Andrew Leach May 29 '14 at 21:46
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    I think that citation is saying that "L" is an acceptable alternative, not that it is the standard. The answer above currently reads to me like it's using the citation to support the latter. – Rupe May 29 '14 at 21:47
  • Can you expand on your answer? As it stands you've written "No, but here's a list of exceptions" 99% exceptions, 1% answer. – kevinaskevin May 30 '14 at 1:22
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No. Yes. Maybe. It depends.

It's always been my understanding with metric measures, that abbreviations for units larger than the base unit measure are capitalized, and units smaller than the base unit are lower case. Dm would be dekameter (ten meters), while dm is decimeter (a tenth of a meter), both based on the unit meter. When not abbreviated, they follow the standard rules for capitalizing words used in written text.

This is also true for prefixes in general, like M for mega- and m for milli-

Non-metric measures like inches, feet and mile follow standard capitalization rules. Since the abbreviations for these never start a sentence, they would never be capitalized.

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    I don't think your understanding is quite right. First, hecto- and kilo- are also multiples but never capitalized. Second, since SI was adopted in 1960, the standard abbreviation for deka- is da-, not D-. And there is the case of micro-, which must be appreviated as µ- to avoid confusion with both mega- and milli-. – choster May 29 '14 at 19:54
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Unit symbols are to be shown/displayed in lower case except for the following reasons.

  1. The first letter of the unit is an uppercase when the name of the unit is derived from the name of the person/scientist. eg: m(meter), s(second), V(Volt), Pa(Pascal)

  2. Liter is represented as 'L' in the United States.

So coming to your case you can use only lowercase for representing the unit. You also need to spell the unit in lowercase unless it is the start of your statement.

I have designed a inch-to-mm converter.

Kilometer is the unit for measuring distance"

If your statement is starting with a unit name, try to spell it out.

The next thing to be remembered is units must always be represented in singular form

c = 25 cm

not

c= 25 cms

Unit symbols are not followed by a period unless it is end of a statement.

"It is 25 cm. long" is the wrong way.
"It is 25 cm long" is the correct way.

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    Traditionally, inches is abbreviated as in. (with the period) to avoid confusion with the word in. – choster May 29 '14 at 20:09
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    That's not correct with regard to units named after people. The general rule is that only the abbreviation is capitalised (so it's 'V' short for 'volt' etc), with the exceptions listed in 200_success's answer. – Rupe May 29 '14 at 20:32

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