Over the years people have used many different systems for measuring things such as length, volume, speed, etc. In many countries the metric system is used. The metric system was developed in France in the late 18th century. Some countries do not use metric units. For example in the United Kingdom, distances on road signs are given in miles, where a mile is about 1.6 kilometres. In the United States of America, weights are measured in pounds instead of kilograms. Under the metric system there are basic units for length, mass, capacity and time. 
Many other units are combinations of these.e.g. Areas can be measured using m^{2}, volumes can use m^{3} and speed can be measured in m/s.
Because many measurements are too large or too small to be conveniently expressed in these basic units, other units are used with prefixes such as kilo or milli.
Things to Measure
Length
Lengths such as distances between towns, perimeters of shapes and lengths of object are measured using the basic unit of the metre.
Length  Units  Comparison 
Basic Unit 
metre (m) 
1 metre = 100 cm 
Other units  millimetre (mm) 
10 mm = 1 cm 
centimetre (cm) 
10 cm = 1 dm 

kilometre (km) 
1 km = 1000 m 
Mass
Mass or as it is commonly known, weight, is a measure of how heavy an object is and is measured using the basic unit of the gram.
Mass  Units  Comparison 
Basic Unit 
gram (g) 
1 gram = 1000 mg 
Other units  milligram (mg) 
1000 mg = 1 g 
kilogram (km) 
1 kg = 1000 g 
Capacity
Capacity or volume is a measure of how much a container can hold or how much space an object takes up and is measured using the basic unit of the litre.
Capacity  Units  Comparison 
Basic Unit 
litre (L) 
1 litre = 1000 mL 
Other units  millilitre (mL) 
1000 mL = 1 L 
Time 
Temperature
The basic unit of temperature is the degree centigrade (or degree Celsius).
The freezing point of water is 0°C and the boiling point of water is 100°C.
Converting Between Units
For measurements too big or too small for the basic units, the following prefixes can be used:


There are other prefixes for even bigger and smaller units, such as mega − , which is a million times bigger than the basic unit and micro , which is a million times smaller than the basic unit.
Examples
What?

How?

Why?

Example

Change metres into centimetres

Multiply by 100

1 m = 100 cm

Change 3.4 m into cm. 3.4 m = 3.4 × 100 cm 
Change millilitres into litres

Divide by 1000

1000 mL = 1 L

Change 500 mL into L 500 mL = 500 ÷ 1000 L 
Change kilograms into milligrams

Multiply by 1 000 000

1 kg = 1 000 000 mg

Change 0.6 kg into mg 0.6 kg = 0.6 × 1 000 000 mg 
Everyday Use of Units
To give an idea of the size of some of the more commonly used units:
1 metre m 
The width of a door 

1 gram g 
A pinch of salt 

1 litre L 
A carton of milk 