Get your writing right – Part 3 – Secondary units and prefixes

Secondary units and prefixes

Sometimes, base and derived SI units are too large or too small to be handled easily. When this happens, modified units (known as secondary units) often prove useful. The size of any secondary unit is defined as a multiple of the base or derived unit. In order to obtain a suitable range for the secondary unit, the base or derived unit may be multiplied by a power of 10. The size of the secondary unit is then indicated by attaching a modifying prefix to the name of the base or derived unit. These prefixes indicate the power of 10 by which the base or derived unit has been multiplied (Table 3).

Table 3 Prefixes and their values

Prefix Symbol Multiple Standard form
yotta Y 1 000 000 000 000 000 000 000 000 1024
zetta Z 1 000 000 000 000 000 000 000 1021
exa E 1 000 000 000 000 000 000 1018
peta P 1 000 000 000 000 000 1015
tera T 1 000 000 000 000 1012
giga G 1 000 000 000 109
mega M 1 000 000 106
kilo k 1 000 103
hecto h 100 102
 deca da 10 101
 deci d  0.1  10-1
 centi c 0.01  10-2
 milli  m  0.001  10-3
 micro i  0.000001  10-6
 nano n  0.000000001  10-9
 pico p  0.000000000001  10-12
 femto f  0.000000000000001  10-15
 atto a  0.000000000000000001  10-18
 zetto z  0.000000000000000000001  10-21
 yocto y  0.000000000000000000000001  10-24

 

Common examples involving the use of prefixes include:

km (kilometre = 1000 m), cm (centimetre 0.01 m), mm (millimetre = 0.001 m) and ìm (micrometre = 0.000001 m) mL or ml (1 millilitre = 0.001 L), ìL or ìl (1 microlitre = 0.000001 L)

 

How big is a billion? Can I use it in scientific writing?

We recommend that you avoid the use of the word billion since it can lead to confusion. Until recently the UK definition was one million million (or 10^12). But in the USA, the billion is defined as one thousand million or (10^9) and this is becoming increasingly the accepted value in the UK.

In most European countries (other than the UK) the billion continues to be defined as one million million.

 

I have read somewhere that the prefixes k and M have different meanings when they are used in computing. Is that right?

This can lead to misunderstandings!

It is common for the size of computer files to be given in units of kb, Mb or Gb (representing kilobytes, megabytes or gigabytes of information). The k, M and G when used in relation to computing actually represent 1024, 1048576 and 1073741824 rather than 100, 1,000,000 and 1,000,000,000. The reasons for the (relatively) small differences arise from numbers in so-called binary form and are beyond the scope of this booklet. You just need to recognise, though, that there are occasions when the misuse of prefixes can lead to confusion.