# 1 Using units of measurement

Before looking at mass, here is a quick reminder of how the SI works from last week.

The SI system uses a combination of base units and prefixes. The base unit that you used in Week 1 was the metre. This is then combined with a prefix, if relevant, to complete the unit. What unit you use will depend upon the size of the measurement that you are taking. For example, it wouldn’t make much sense to measure the length of a finger in kilometres! The most common everyday prefixes are:

- ‘kilo-’ a thousand
- ‘centi-’ a hundredth
- ‘milli-’ a thousandth.

This gives the following units of length:

- kilometre = 1000 metres
- centimetre = 0.01 metres
- millimetre = 0.001 metres

This idea is extended to all the SI units, as you’ll see in the next sections on mass.