Numbers, units and arithmetic
Numbers, units and arithmetic

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Numbers, units and arithmetic

2.1 Which units to use

It is important to choose appropriate units, both to have a sense of the size you are talking about and also to avoid having to deal with very large or very small numbers.

For example using the UK decimal system, where £1 = 100p (one pound is equivalent to 100 pence), you would probably use pence for prices less than a pound: 50p rather than £0.50 and pounds for larger amounts, e.g. £2.50 instead of 250p.

For distance, mass and liquid measures, the metric system of units is often used. For example, the distance a cyclist cycles in a day is likely to be given in kilometres (sometimes written as the symbol km), a person's height in metres (m) and a waist measurement in centimetres (cm). The prefixes ‘kilo’ and ‘centi’, together with ‘deci’ and ‘milli’, are used throughout the metric system. Some common prefixes are shown in Table 1.

Table 1: Prefixes used in the decimal system

micro-μone millionth0.000001
milli-mone thousandth0.001
centi-cone hundredth0.01
deci-done tenth0.1
mega-Mmillion1 000 000
giga-Gthousand million1 000 000 000

So 1 centimetre = metre or 1 cm = m.

This is the same as saying there are 100 centimetres in 1 metre or 100 cm = 1 m.

For mass, kilograms (kg), grams (g) or milligrams (mg) are usually used and a metric tonne is used for very large masses (1000 kg). The word ‘mass’ is scientifically more precise than ‘weight’, although people usually talk about weight rather than mass. For liquid capacity, litres (l), decilitres (dl), centilitres (cl) or millilitres (ml) are used.


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371