Just as there are right and wrong ways of spelling, there are right and wrong ways of writing with metric units. However the rules are consistent and easy. (See also UKMA’s Style Guide at this link).
Symbols and Abbreviations
Symbols and abbreviations are not the same. Symbols such as %, £, € or $ are international. Abbreviations such as “HGV”, “PL” or “LKW” are language specific (English, French and German respectively) but all refer to heavy goods vehicles. If an international symbol exists it is confusing to use an abbreviation instead – for example using “p.c.” for percent can be confused with postcard or even police constable or personal computer – a symbol such as % is not open to misunderstanding.
As a result of international agreement standard symbols have been defined for metric units regardless of language. Thus kilometre is written as “km” (using the Roman alphabet) in all languages even though the full word is spelt differently, for example:
| Language | symbol | spelling |
|---|---|---|
| English | km | kilometre |
| French | km | kilomètre |
| German | km | kilometer |
| Italian | km | chilometro |
| Spanish | km | kilómetro |
| Mandarin | km | 千米 |
| Arabic | km | كيلومتر |
In Britain some people are confused by the fact that abbreviations, with more than one variant, are accepted for imperial units. For example miles are abbreviated variously as “mls”, “mi” and very confusingly on British motorway signs as “m” where they could also be read as metres. Feet may be abbreviated as “ft” or with ‘ (also the international symbol for minute of an arc). Ounces are abbreviated oz after the Italian word onza.
Abbreviations for metric units (like ‘mtrs’) are non-standard and should be not used. Symbols like ‘m’ should be always used.
Prefixes follow the same rules. The standard symbols for prefixes should always be used if a unit is not being written longhand. A symbol such as kilowatt (kW) is formed from the symbol for prefix (k-) and symbol for watt (W).
Plurals and Capitals
Like other symbols, metric symbols should not have an ‘s’ added to indicate a plural. Thirty percent is written as ‘30%’ and not ‘30%s’. Similarly metric symbols do not change in the plural – thirty kilograms is written as ’30 kg’ not ’30 kgs’.
There should also be a space or half space between the number and symbol. For example ‘2 MW’ not ‘2MW’. A non-breaking space can be entered using the Ctrl-Shift-Space key combination.
It is important to pay attention to whether small or capital letters are used. The same letter can have quite different meanings depending on whether it is a capital or not. For example
‘k’ is the symbol for the prefix kilo- (one thousand)
‘K’ is the symbol for kelvin, the base unit of absolute temperature‘m’ is the symbol for the metre and for the prefix milli- (one thousandth)
‘M’ is the symbol for the prefix mega- (one million)
When describing temperatures in degrees Celsius, always use the ° degree symbol.
‘30 °C’ indicates a temperature of thirty degrees
’30 C’ indicates an electric charge of thirty coulombs!
Note that many metric units are named after scientists. However the unit names, when written longhand, should always begin in lower case – unlike the upper case used with the scientist’s name – apart from the exception of the degree Celsius. Some examples are given below:
| Unit symbol | Unit name | Scientist name |
|---|---|---|
| A | ampere | André Ampère |
| C | coulomb | Charles-Augustin de Coulomb |
| °C | degree Celsius | Anders Celsius |
| F | farad | Michael Faraday |
| Gy | gray | Louis Harold Gray |
| H | henry | Joseph Henry |
| Hz | hertz | Heinrich Rudolf Hertz |
| J | joule | James Prescott Joule |
| K | kelvin | Lord Kelvin |
| Ω | ohm | Georg Ohm |
| N | newton | Isaac Newton |
| Pa | pascal | Blaise Pascal |
| S | siemens | Werner von Siemens |
| T | tesla | Nikola Tesla |
| V | volt | Alessandro Volta |
| W | watt | James Watt |
UK Metric Association 