Writing Guide Home SI Units  

Science uses the International System of Units know as SI Units which comes from the French Système International d'Unités.

The following are examples of commonly used SI Units and the symbols used:

Measurement
SI Unit
Symbol
Distance
kilometres
km
metres
m
centimetres
cm
millimetres
mm
Mass
kilograms
kg
grams
g
milligrams
mg
Volume
decimetres cubed
dm3
centimetres cubed
cm3
Area
metres squared
m2
centimetres squared
cm2
millimetres squared
mm2
Energy
joule
J
kilojoule
kJ
Pressure
Pascal
Pa
kiloPascal
kPa
Concentration
moles
mol
millimoles
mmol
Time
seconds
s
minutes
min
hours
h
days
d

In Biology, we use % as an indicator of concentration We would refer to a 1% solution for example in the following way:

1 % xxxxxxxx (w/v) would be 1g of xxxxxxxx powder for a 100 cm3 solution
1% yyyyyyyy (v/v) would be 1 cm3 of yyyyyyyy liquid for a 100 cm3 solution

You will have noticed that SI Units have certain prefixes and you can add prefixes to increase or decrease the size. The prefixes are shown below with the factor and symbol.

Name
Factor
Symbol
pico
10-12
p
nano
10-9
n
micro
10-6
µ
milli
10-3
m
centi
10-2
c
deci
10-1
d
deca
101
da
hecto
102
h
kilo
103
k
mega
106
M
giga
109
G
tera
1012
T

All you need to do is add the prefix in front of the SI Unit and it will change the size by the factor. For example add pico in front of metre and it become a picometre which is 10-12 metres or 0.000000000001 metres. The important thing to remember is that you can’t combine prefixes, you can’t have millikilogram for example.

The correct use of SI Units:

Rule
Do
Don't
Abbreviations
Use correct SI Units
Don’t use sec for seconds or cc for cubic centimetre for example
Clear units

12 mm x 5 mm

35 °C to 40 °C

5 g ± 0.1 g

12 x 5 mm

35 ˚C – 40 °C or 35 – 40 °C

5 ± 0.1 g

Full stops - No full stop after a symbol unless it is at the end of a sentence.

The cell is 50 µm wide.

The width of the cell is 50 µm.

The cell is 50 µm. wide.
Mass
Describe g or kg as mass
Describe g or kg as weight
Mixed information
The volume of gas produced was 12 cm3 min-1
12 cm3 of gas / min-1
Mixed symbols and names

1.5 g cm-3

1.5 g / cm3

1.5 grams cm-3

1.5 g / cubic centimetre

Concentration
0.2 mol dm-3

0.2 M (M is no longer used to describe molarity)

0.2 mol (moles should always be given by volume)

Plural
10 cm
10 cms

As seen above you can use " / " or factorise the second symbol instead of using the word "per". For example, "The rate of reaction was 5 cm3 per second" would become:

"The rate of reaction was 5 cm3 / s"
or
"The rate of reaction was 5 cm3 s-1"