1.3 Measuring in millimetres and centimetres

When you’re measuring an item, you need to decide whether to measure it in millimetres, centimetres or metres. Often, your decision will be based on the size of the item, why you are measuring it and how accurate you need the measurement to be.

When you measure an item, you can actually express the same measurement in different ways. To help you to do this, you need to know some metric measure facts:

Facts

10 millimetres = 1 centimetre

100 centimetres = 1 metre

1 000 metres = 1 kilometre

Starting with the smallest, metric units of length are millimetres (mm), centimetres (cm) and metres (m).

Kilometres (km) are used to measure distance.

Example: Writing measurements in millimetres and centimetres

You can express a measurement in millimetres, centimetres or a combination of both. Look at the ruler below. What measurement is the arrow pointing to?

Figure 8 Measuring a pen

Method

The numbers displayed on the ruler represent centimetres. Each line in between each whole centimetre is a millimetre; ten millimetres is equal to one centimetre. So you can say that the arrow is pointing to:

Extract

1 cm 7 mm

However, you could write it all in centimetres. The length is one whole centimetre plus seven additional millimetres, so you would write:

Extract

1.7 cm

Note how the decimal point separates the number of centimetres from the number of millimetres. Alternatively you could write this measurement in millimetres. One centimetre equals ten millimetres, so:

Extract

1 cm 7 mm = 10 mm + 7 mm = 17 mm

Now try the following activity.

Activity 4: Writing measurements in different ways

Complete the gaps in the table to show the same measurement written in three different ways. The first one has been done for you. Remember to check your answers.

Measurements

Centimetres and millimetres	Centimetres	Millimetres
1 cm 7 mm	1.7 cm	17 mm
8 cm 9 mm	(A text entry box would appear here, but your browser does not support it.)	(A text entry box would appear here, but your browser does not support it.)
(A text entry box would appear here, but your browser does not support it.)	9.4 cm	(A text entry box would appear here, but your browser does not support it.)
(A text entry box would appear here, but your browser does not support it.)	(A text entry box would appear here, but your browser does not support it.)	63 mm
12 cm 6 mm	(A text entry box would appear here, but your browser does not support it.)	(A text entry box would appear here, but your browser does not support it.)
(A text entry box would appear here, but your browser does not support it.)	(A text entry box would appear here, but your browser does not support it.)	105 mm
(A text entry box would appear here, but your browser does not support it.)	20.1 cm	(A text entry box would appear here, but your browser does not support it.)

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Answer

Measurements

Centimetres and millimetres	Centimetres	Millimetres
1 cm 7 mm	1.7 cm	17 mm
8 cm 9 mm	8.9 cm	89 mm
9 cm 4 mm	9.4 cm	94 mm
6 cm 3 mm	6.3 cm	63 mm
12 cm 6 mm	12.6 cm	126 mm
10 cm 5 mm	10.5 cm	105 mm
20 cm 1 mm	20.1 cm	201 mm

◄Contents►Next: 1.4 Converting units

OpenLearn - Everyday maths 1 (Northern Ireland)
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