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 m2, volumes can use m3 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

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
1 metre = 1000 mm

Other units millimetre (mm)

10 mm = 1 cm
1000 mm = 1 m

centimetre (cm)

10 cm = 1 dm
100 cm = 1 m

kilometre (km)

1 km = 1000 m
1 km = 100 000 cm



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 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 is measured using the basic unit of the second. There are no other special metric units for time and the other units used include minutes, hours, days etc.


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:


Multiple of 10
Smaller than the basic unit
× 11000
× 10-3



× 1100
× 10-2
× 110
× 10-1
The basic unit
× 10
× 10
D or da
× 100
× 102
Bigger than the basic unit
× 1000
× 103

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.


Change metres into centimetres
Multiply by 100
1 m = 100 cm

Change 3.4 m into cm.

3.4 m = 3.4 × 100 cm
340 cm

Change millilitres into litres
Divide by 1000
1000 mL = 1 L

Change 500 mL into L

500 mL = 500 ÷ 1000 L
0.5 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
600 000 mg

Everyday Use of Units Rates.jpg

To give an idea of the size of some of the more commonly used units:


Examples (approximation)

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

Mass (weight)

1 gram g
1 kilogram kg
1 Tonne T

A pinch of salt
Two packets of butter
A small truck


1 litre L
1 millilitre mL

A carton of milk
5 mL is a small teaspoonful of liquid