SI (pronounced “ess-EYE”) is the initialism for the Système international d’unités, the modernized version of the metric system that the US and other nations have agreed to use. (Do not abbreviate it as “S.I.”)
This list is provided to point out the correct way to use the metric system and to show many of the incorrect examples of its usage that may be given on package labels and in other printed matter. These correct ways to use the SI and other related units are set by the international standards that define the SI.
Important points
- The short forms for metric units (such as mm for millimetre) are symbols, not abbreviations.
- As such, metric symbols never end with a period unless they are the last word in a sentence.
- Correct: 20 mm, 10 kg
- Incorrect: 20 mm., 10 kg.
- For the same reason, symbols always are written in the “singular” form (even when more than one is meant).
- Correct: 1 mm, 500 mm, 1 kg, 36 kg
- Incorrect: 500 mms, 36 kgs
- But, it is correct to pluralize unit names: 25 kilograms, 250 millilitres
- Likewise, symbols are case-sensitive, so the meaning of a symbol can be changed if you substitute an uppercase letter for a lowercase letter (because they’re different shapes).
- Correct: mm (for millimetre, which means 1/1000 of a metre)
- Incorrect: MM or Mm (M is the symbol for the prefix mega-, which means one million; a megametre is a million metres)
- Symbols should be preceded by digits and a space must separate the digits from the symbol.
- Correct: It was 300 mm wide. The millimetre width was given.
- Incorrect: It was 300mm wide. The mm width was given.
- The symbol for a compound unit that is a product of two units is indicated by a multiplication dot (dot operator) or by a space. See How can I type unit symbols such as m2, °C, N⋅m, and µm? in the FAQ for guidance on typing symbols.
- Correct: kW⋅h or kWh (for kilowatt-hours)
- Incorrect: kWh (do not write symbols next to each other)
- The symbol for a compound unit that is a quotient of two units is indicated by a forward slash (solidus) or by a negative exponent.
- Correct: km/h or km⋅h-1 (for kilometres per hour)
- Incorrect: kmph or kph (do not use p as a symbol for “per”; p is the symbol for the prefix pico-, which means one trillionth)
- But, it is correct to say or write “kilometres per hour”.
- The points above are not subject to a house writing style nor limited to scientific/technical contexts, but are the only way to properly use metric units in any writing context. The metric system does not distinguish between “scientific contexts” and “everyday contexts”; it is “à tous les temps, à tous les peuples” (“for all time, for all peoples”).
Examples of incorrect usage
| Correct Usage | Incorrect Usage | For |
|---|---|---|
| km | Km, km., KM, kms, K, k | kilometre |
| km/h | kph, kmph, KPH, km/hr | kilometre per hour |
| °C | C, deg C, ° C, C° | degree Celsius |
| m | M, m., mtr, mts, mtrs, MTR, MTS, MTRS | metre |
| L, l | L., l., ltr, ltrs | litre |
| mL, ml | ML, Ml, mL., ml., mls | millilitre |
| kg | KG, KG., Kg, Kg., kgr, kgs, kilo | kilogram |
| h | hr, hrs, HR, h., HR., HRS. | hour |
| min | m, mins, M, m., MIN, MINS | minute |
| s | sec, S, SEC, sec., s., S. | second |
| mm | Mm, mm., MM | millimetre |
| m2 | sq m, sqm, sq. m., sq. mtr. | square metre |
| kW⋅h, kWh | kWh, kwh, Kwh, KWH, kWH, kw.h, kW.h, kw/h, KW/H | kilowatt-hour |
| g | G, G., g., gr, gm, gms, GR, GM, GRM, grms | gram |
| µg | mcg*, ug* | microgram |
| cm3 | cc, cu cm | cubic centimetre |
| kHz | KHz, KHZ, Khz | kilohertz |
| MHz | MHZ, Mhz | megahertz |
| hPa | HPa, HPA, Hpa, mb | hectopascal |
| kPa | KPa, KPA, Kpa | kilopascal |
| K | °K, deg K | kelvin |
Notes
* Because the symbol “µg”, when handwritten, looks similar to “Mg” (megagram), which is often contextually interpreted as “mg” (milligram), and is therefore a frequent cause of overdoses, the abbreviation “mcg” is preferred in the medical field in the US (see the Joint Commission recommendations). For a similar reason and also the reason of being difficult to type (see How can I type unit symbols such as m2, °C, N⋅m, and µm?), the symbol “ug” is sometimes used, especially in other countries, due to its similarity in appearance to “µg”. Despite their prevalence, these abbreviations are still incorrect.
Explanations
The spelling of metre and litre is the official and standard one, and that of meter and liter is non-standard. In the US, the meter and liter spellings are commonly used despite their unofficial and non-standard status, even though metre and litre are just as valid and legal; the English spelling used by all other nations and international organizations (including the BIPM itself and ISO) is exclusively metre and litre.
In a strict sense, spelling and pronunciation are matters of language and are not set by the international standards that define the SI. However, the SI is officially published in the English and French languages, so the spelling of SI units in these languages follows the official specification. Official specification notwithstanding, rules and patterns nevertheless exist within languages which guide spelling and pronunciation. As such, in keeping with the pronunciation of the other metric units involving prefixes, which all accent the first syllable, the logically consistent and proper pronunciation of the word kilometre is KILL-oh-meet-ur, not kill-AHM-it-ur. This is similar to the word nanometre, which is pronounced NAN-oh-meet-ur, not nan-AHM-it-ur (see How do you pronounce “kilometre”?).
The symbol for litre may be either a uppercase “ell” (L) or a lowercase “ell” (l); both are correct. In the US, Canada, and Australia, the uppercase “ell” (L) is preferred since it is easily distinguished from the digit one (1) and the uppercase “eye” (I), but most other nations use the lowercase “ell” (l) to adhere strictly to the standard of uppercase symbols being reserved for units derived from the names of people (see the story of the fictional Claude Litre).
A list of the SI/metric units and symbols, plus more details on their use, is given in USMA’s Guide to the Use of the Metric System.
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