SI (pronounced ess-EYE) is the symbol for the Système International d’Unités, the modernized version of the metric system that the U.S. 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 SI are set by the international standards that define the SI.
Important in SI-metric:
- The short forms for SI units (such as mm for millimeter) are symbols, not abbreviations.
- SI symbols never end with a period unless they are the last word in a sentence.
- RIGHT: 20 mm, 10 kg
- WRONG: 20 mm., 10 kg.
- SI symbols should be preceded by digits and a space must separate the digits from the symbol.
- RIGHT: It was 300 mm wide. The millimeter width was given.
- WRONG: It was 300mm wide. The mm width was given.
- Symbols always are written in the singular form (even when more than one is meant).
- RIGHT: 1 mm, 500 mm, 1 kg, 36 kg
- WRONG: 500 mms, 36 kgs
- BUT: It is correct to pluralize written-out metric unit names: 25 kilograms, 250 milliliters
- The symbol for a compound unit that is a quotient of two units is indicated by a solidus or by a negative exponent.
- RIGHT: km/h or km·h-1 (for kilometers per hour)
- WRONG: kmph or kph (do not use p as a symbol for “per”.)
- BUT: It is correct to say or write “kilometers per hour”.
- The meaning of an SI symbol can be changed if you substitute a capital letter for a lower case letter.
- RIGHT: mm (for millimeter, which means 1/1000 of a meter)
- WRONG: MM or Mm (M is the prefix for mega, which means one million; a megameter is a million meters)
Examples of incorrect SI-metric usage:
| Correct Usage | Examples of Incorrect Usage | For |
|---|---|---|
| km | Km, km., KM, kms, K, k | kilometer |
| m | M, m. | meter |
| mm | Mm, mm., MM | millimeter |
| L or l | L., l. | liter |
| mL or ml | ML, Ml, mL., ml., mls | milliliter |
| kg | KG, KG., Kg, Kg., kgr, kgs, kilo | kilogram |
| g | G, G., g., gr, gm, GR, GM, GRM, grms | gram |
| µg | mcg1 | microgram |
| h | hr, hrs, HR, h., HR., HRS. | hour |
| s | sec, S, SEC, sec., s., S. | second |
| cm3 | cc | cubic centimeter |
| km/h | KPH, kph, kmph, km/hr | kilometer per hour |
| kHz | KHz, KHZ, Khz | kilohertz |
| MHz | MHZ, Mhz | megahertz |
| hPa | HPa, HPA, Hpa, mb | hectopascal |
| kPa | KPa, KPA, Kpa | kilopascal |
| °C | C, deg C, ° C, C° | degree Celsius |
| K | °K, deg K | kelvin |
1Because the handwritten symbol “µg” looks almost exactly like “mg” and is therefore a frequent cause of overdoses, the abbreviation “mcg” is preferred in the medical field. See The Joint Commission recommendations.
Some explanations:
The spellings of meter and metre [or liter and litre] are both correct. In the U.S., the meter and liter spellings are used most often; but the English-spellings used in most other nations are metre and litre.
In a strict sense, spelling and pronunciation are matters of language and are not set by the international standards that define SI. But, in keeping with the pronunciation of the other SI units involving prefixes, which all accent the first syllable, the USMA-preferred pronunciation of the word, kilometer, is KILL-oh-meet-ur, (not kill-AHM-it-ur).
The symbol for liter (or litre) may be either a capital el (L) or a lowercase el (l); both are correct. In the U.S., Canada, and Australia, the capital el (L) is preferred, but most other nations use the lowercase el (l).
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.