Get your writing right – symbols & units

If you’re writing a scientific paper, an essay or a report, you want to get it right.

In science, it is important that we are able to present results in a way that minimises confusion. Scientists therefore adopt a common system for presenting data that is widely used and understood.

This guide will:

– help you remember the units and symbols that you may meet in your science courses

– remind you of the importance of using units correctly.


Part 1 – SI base units

For many years in the UK, teaching in schools and colleges has encouraged the use of SI units. (SI stands for Système International d’Unités or International System of Units) SI units are a coherent system of units comprising seven base units on which all other units are based.


Table 1 SI base units


Physical quantity Name Symbol
Length Metre M
Mass Kilogram Kg
Time Second s
Amount of substance Mole Mol
Thermodynamic temperature Kelvin K
Electric current Ampere A
Lumious intensity Candela cd


*Note the spelling of gram rather than gramme.


So, what should I learn from table 1?

Well, there are 2 main points

  1. The name for a unit always starts with a small (or lower case) letter even though they may have been named after a person e.g. the kelvin is named after Lord Kelvin.
  2. The symbol for a unit starts with a capital letter only if it is named after a person e.g. A is the symbol for ampere. If the symbol for a unit is not named after a person it always starts with a small (or lower case) letter – for example, the symbol for second is s (not S!).


There is one exception to the rule about symbols not starting with a capital letter (unless they are named after a person). This exception involves the symbol for the litre. The symbol for the litre is not named after a person and technically its correct symbol should be the letter l (lower case). But you can see how easily this can be confused with the number 1, depending on the typeface used. So we can use the capital letter L as the symbol for the ‘litre’. This means that the symbol for the litre can either be l or L and both forms are acceptable.