Every measurement must have a size (eg 2.7) and a unit (eg metres or ºC). Sometimes, there are different units available for the same type of measurement. For example, ounces, pounds, kilograms and tonnes are all used as units for mass.

To reduce confusion, and to help with conversion between different units, there is a standard system of units called the SI units which are used for most scientific purposes.

These units have all been defined by experiment so that the size of, say, a metre in the UK is the same as a metre in China.

The seven SI base units are:

All other units can be derived from the SI base units.

For example, area is measured in square metres (written as m2) and speed is measured in metres per second (written as ms–1).

It is not always appropriate to use a full unit. For example, measuring the width of a hair or the distance from Manchester to London in metres would cause the numbers to be difficult to work with.

Prefixes are used to multiply each of the units. You will be familiar with centi (meaning 1/100), kilo (1000) and milli (1/1000) from centimetres, kilometres and millimetres.

The most common prefixes you will encounter are:

Activity 1

Which SI unit and prefix would you use for the following quantities?

1. The time between heart beats

2. The length of a leaf

3. The distance that a migratory bird travelled each year

4. The width of a cheek cell

5. The mass of a rabbit

6. The mass of iron in the body

7. The volume of the trunk of a large tree

Activity 2

Choose the most appropriate unit, and estimate the size of each of the following.

1. The mass of an elephant

2. The mass of an earthworm

3. The volume of water in a teardrop

4. The volume of water in a pond

5. The time taken for a sunflower to grow

6. The temperature difference between the blood in the heart and in the ear on a cold day

7. The width of a hair

8. The length that your fingernails grow each day

Activity 3

Put the following in order of size:

height of an elephant; length of DNA strand; width of a hair; height of a tree; width of a sodium ion; length of a nerve cell; length of a heart; width of a red blood cell; size of a virus; length of a finger; length of a mosquito; length of a human digestive system; width of a field; length of a water molecule.