Get your writing right – Part 5 – Using SI units and their symbols

In an attempt to avoid confusion, a number of rules or conventions are followed when using units and symbols.

This section deals with some of these conventions and you will see that we list 12 different points that you should consider.

  1. Symbols consist of capital letters when the units that they represent are named after a person.

For example, the joule (symbol J) is named after James Joule – but note the unit name does not have an initial capital letter.

Example An energy content of 40 joule = 40 J (not 40 j)

A force of 50 newton = 50 N (not 50 n)


  1. Insert a space between the number and unit.

Example 75 kg (not 75kg)

The exception to this is when a percentage is being expressed.

Example 75% (not 75 %)


  1. Temperature

The symbol for temperature in celsius (°C) is preceded by a space when expressing values for celsius temperatures.

Example 30.4 °C (not 30.4°C or 30.4° C)

The degree sign is omitted when recording temperatures in the SI unit (kelvin)

Example The melting point of water is 273.15 K (not 273.15 °K)


  1. Symbols must be written in their singular form.

In some cases it may not be possible to avoid the use of the ‘s’ after the unit especially in written text. However, there should never be an ‘s’ after the symbol unless it stands for second!

Example 2.4 mm (not 2.4 mms); 50 W (not 50 Ws); 50 kg (not 50 kgs)


  1. No full stop should be inserted after the symbol (except at the end of a sentence!)

Example six milligram of aspirin = 6 mg aspirin (not 6 mg. aspirin)


  1. When two or more unit symbols are combined to create a derived unit symbol they should be separated by a space.

Example 1 L min-1 (not 1 Lmin-1)


  1. No space is left between a prefix (that indicates a power of 10) and the symbol to which it applies.

This rule is particularly important in avoiding confusion between m (meaning milli) and m (meaning metre).

Example One millisecond = 1 ms

One metre per second = 1 m s-1


  1. Compound prefixes should not be used – use only one multiplying prefix.

Example 10-9 m = 1 nm (not 1mìm)


  1. A combination of prefix and symbol for a unit is regarded as a single symbol.

This is often a source of confusion. In mathematics, if we were to come across the expression I dm3 we might assume that it means 1 x d x m x m x m whereas in the sense of a unit 1 dm3 is the same as 1 (dm)3.

This might become clearer if we look at the following:

Example 1 litre = 1 dm3 = 1 (dm)3 = 10-3 m3 (not 10-1 m3)

1 cm3 = 1 (cm)3 = 10-6 m3 (not 10-2 m3)

1 cm3 = 10-3 dm3

(See Appendix 2 for more information on units of volume in common usage).


  1. To help with the reading of numbers it is better not to use commas. This avoids confusion with the (occasional) use of the comma to denote a decimal point. Internationally the comma is often used in place of a decimal point and this can lead to confusion.

Examples a. The number 123456789.23 may be written as 123 456 789.23 i.e. with a space separating groups of three digits (but should not be written as 123,456,789.23)

b. When writing a four-digit number the inclusion of a space is optional e.g. 1 000 or 1000.

  1. The solidus (/) is discouraged in favour of the negative index when writing symbols of reciprocal units.

Example It is preferable to write N m-2 rather than N/m2

The solidus must never be used more than once in any unit.

Example The molar gas constant, R = 8.314 J K-1 mol-1 (not 8.314 J/K/mol)

  1. Symbols are used only when they are preceded by a numerical value. Symbols should not be used as abbreviations within a sentence.

Examples a. It is sold by the cubic metre (not it is sold by the m3)

b. There are 106 mm in 1 km (but not there are many mm in a km)


Spot the errors!

Let’s look at an example of text and see if we can spot any errors in the way that units and symbols have been used. If you were asked to re-write the following piece of text (shown in italics below) how many changes would you make?

…In the United States, energy production from natural gas released about 5.5 billion tonnes of waste in 1994. Natural gas fires and explosions are also significant risks. A single mile of pipeline three feet in diameter at a pressure of 1000 pounds per square inch (psi) contains the equivalent of two-thirds of a kiloton of explosive energy; a million miles of such pipelines lace the earth.

(The above passage was taken from an article, ‘The need for nuclear power: viewpoint on the world’s challenging energy future’, by Richard Rhodes and Denis Beller published in 2000 in volume 42 (part 2) of the International Atomic Energy Agency Bulletin, pages 43-50).

Compare your answers with the suggestions given in Answers 2 Published article

Further information on the correct use of units is available (see Appendix 3).