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 
dm^{3} 
centimetres cubed 
cm^{3} 
Area 
metres squared 
m^{2} 
centimetres squared 
cm^{2} 
millimetres squared 
mm^{2} 
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 cm^{3} solution
1% yyyyyyyy (v/v) would be 1 cm^{3} of yyyyyyyy liquid for a 100 cm^{3} 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 
10^{1} 
da 
hecto 
10^{2} 
h 
kilo 
10^{3} 
k 
mega 
10^{6} 
M 
giga 
10^{9} 
G 
tera 
10^{12} 
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 cm^{3} min^{1} 
12 cm^{3} of gas / min^{1} 
Mixed symbols and names 
1.5 g cm^{3}
1.5 g / cm^{3}

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 cm^{3} per second" would become:
"The rate of reaction was 5 cm^{3} / s"
or
"The rate of reaction was 5 cm^{3} s^{1}"
