The metric system is a system of units for measuring quantities such as the length of a line or the weight of a person that were covered in the previous few topics.
The 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 and lengths are usually given using the British Imperial System which uses yards instead of metres and pounds instead of kilograms.
Under the metric system called the International System of Units, there are basic units for length, mass, capacity and time. Many other units are combinations of these. Because many measurements are too large or small to be conveniently expressed in these basic units, the size of the basic unit can be increased or decreased by multiples of 10, and this is shown by a prefix such as kilo or milli.
The Basic Units
Length The basic unit for length is the metre (symbol m).
 The basic unit for mass is the gram (symbol g).
 Weight is commonly used to describe mass.
 A tonne is equal to 1000 kilograms. (This is called a metric ton).
The basic unit for capacity or liquid volume is the litre (symbol L).
There are two systems for telling the time, the 24 hour clock and the analogue (am and pm) system. These are covered in detail in topic 10 on Time

 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.
Prefixes
For measurements too big or too small for the basic units, the following prefixes can be used:


There are other proefixes 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.
To give an idea of the size of some of the more commonly used units:
1 metre m 1 kilometre km 1 centimetre cm 
The width of a door Three laps of a running track The width of a $1 coin 

1 gram g 1 kilogram kg 
A pinch of salt Two packets of butter 

1 litre L 1 millilitre mL 
A carton of milk 5 mL is a small teaspoonful 