The metric system is a system of units for measuring quantities such as the length of a line or the weight of a person. 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).
Time

For an exciting and fun practice with speed, distance, time and bearings −
Prefixes
For measurements too big or too small for the basic units, the following prefixes can be used:


For even bigger or smaller measurements the following prefixes can be used.
tera  
giga  
mega 
micro 
nano  
pico  
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 
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.
Time
There are two systems for telling the time.
am and pm
 Times during the first 12 hours of the day (during the morning between midnight and midday) are followed by the letters am
 e.g. 3.42 am means 42 minutes after 3 o'clock in the morning.
 Times during the last 12 hours of the day (during the afternoon and evening between midday and midnight) are followed by the letters pm
 e.g. 5.26 pm means 26 minutes after 5 o'clock in the afternoon.
24hour clock
The hours of the day are numbered from 0 to 24, starting with 0 as the first hour after midnight.
e.g. 0047 means 47 minutes past midnight 0800 means 8 o'clock in the morning 1530 means half past three in the afternoon 2307 means 7 minutes after 11 o'clock at night

Melbourne, Australia at about 2030 in winter 
Special times
Midday is written 1200 or 12.00 pm, noon or midday
Midnight is written 0000 or 12.00 am or midnight.