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Measuring length

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Measurement, area & perimeter

Measuring length

You can measure length in centimetres, metres and kilometres. Choosing the right unit and rounding to the nearest whole number helps you describe how long things are.

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Estimating and measuring

When you estimate, you make a good guess ā€” it does not need to be exact. When you measure with a ruler or a stick, the measurement needs to be exact. For example, a line might be estimated at 10 cm and measured at 11 cm.

Choosing the right unit

ā€¢ Centimetres (cm) are for small lengths, like your foot, a pencil or a phone. ā€¢ Metres (m) are for room-sized lengths, like a door or a football pitch. ā€¢ Kilometres (km) are for long distances, like a marathon or a long journey.

How the units connect

The units join together neatly: ā€¢ 100 cm = 1 metre ā€¢ 1000 m = 1 kilometre So 7 m = 700 cm, and half a metre = 50 cm.

Rounding a measurement

Rounding gives a simpler number that is close to the real one. A table that is 108 cm long rounds to 110 cm to the nearest 10 cm, and to 100 cm to the nearest 100 cm.

Worked examples

Estimating then measuring

  1. Estimate the length of the line first: about 10 cm.
  2. Measure it exactly with a ruler: 11 cm.
  3. Compare the estimate and the measurement.

Answer: The estimate was 10 cm and the measurement was 11 cm ā€” a close estimate.

Changing metres to centimetres

  1. Remember 100 cm = 1 metre.
  2. You have 7 metres, which is 7 lots of 100 cm.
  3. Multiply: 7 Ɨ 100.

Answer: 7 m = 700 cm.

Choosing a unit

  1. You want to measure the distance of a marathon race.
  2. A marathon is a very long distance.
  3. Long distances are measured in kilometres.

Answer: You would measure a marathon in kilometres.

Rounding to the nearest 10 cm

  1. The table is 108 cm long.
  2. The nearest tens are 100 cm and 110 cm.
  3. 108 is closer to 110.

Answer: 108 cm rounds to 110 cm to the nearest 10 cm.

Where you see this in real life

  • Measuring your shoe or pencil in centimetres.
  • Measuring the height of a door in metres.
  • Talking about the distance of a long car journey in kilometres.

Key words

Centimetre (cm)
A small unit of length. 100 cm make 1 metre.
Metre (m)
A unit of length for room-sized things. 1 m = 100 cm.
Kilometre (km)
A unit of length for long distances. 1 km = 1000 m.
Estimate
A sensible guess that does not need to be exact.
Rounding
Changing a number to a nearby, simpler number.
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