Lesson 1 of 8 · Grades 3 to 5
Length in metric units
Metric length is built on tens, so it is easy to move between units. In this lesson you will read a ruler in millimetres and centimetres and choose the right unit for the thing you are measuring, from a ladybug to a road trip.
Ten small steps to a centimetre
A millimetre (mm) is the smallest unit on an ordinary ruler. Ten of those small steps make one centimetre (cm), so 1 cm = 10 mm. To measure a short object, line up one end with the zero mark and read the number at the other end. A paperclip is about 3 cm long. A ladybug is about 7 mm long.
The ruler below is marked in centimetres with a small tick for every millimetre. The crayon runs from 2 cm to 7 cm, so it is 5 cm long. Always start at zero, or note the start mark and subtract.
From centimetres to metres
One hundred centimetres make one metre (m): 1 m = 100 cm. A door is about 2 m tall, a kitchen table is about 1 m long, and a new pencil is about 19 cm. Because each centimetre is 10 mm, a metre is also 1,000 mm. A metre stick often has millimetre marks along one edge and centimetre numbers along the other, and both scales measure the same stick.
Kilometres for long distances
A kilometre (km) is 1,000 metres. You use kilometres for distances you would walk, cycle or drive rather than measure with a ruler. A short walk to the shops might be 1 km, a run around a park might be 3 km, and the drive to the next town might be 12 km. Kilometres are the metric cousin of miles, and one kilometre is a little more than half a mile.
Changing to a smaller unit multiplies by ten at every step. Four metres is 400 centimetres, and 4,000 millimetres. Each step adds zeros, because a smaller unit needs more of them to cover the same length.
Choosing a sensible unit
A good measurement uses a unit that suits the object. Lengths you hold are measured in millimetres or centimetres, lengths you walk across in metres, and lengths you travel in kilometres. Estimation helps too: a wall is about 3 m long, so a measurement of 300 m should make you stop. When an estimate and a measurement are far apart, check the unit before the arithmetic.
Watch out for this
The wrong unit can make a sentence silly. The classroom is not 8 mm wide, and the road to school is not 500 cm long. Compare the number with the unit before you say them together: 8 m is reasonable for a room.
Writing a length in two units
A length does not always land exactly on a whole unit. When a plank is 3 m and 45 cm long, it is written 3 m 45 cm. Both parts matter. Adding them into one unit gives 345 cm, because 3 m is 300 cm and 300 + 45 = 345. The same length is 3,450 mm.
Work in the unit that makes the arithmetic easiest. If two lengths use different units, change both to the smaller one first, then compute.
Worked example
A plank is 3 m 45 cm long. Write the length in centimetres, then in millimetres.
Step 1: change the metres to centimetres. 1 m = 100 cm, so 3 m = 300 cm.
Step 2: add the centimetres. 300 cm + 45 cm = 345 cm.
Step 3: change the centimetres to millimetres. 1 cm = 10 mm, so 345 cm = 3,450 mm.
Step 4: check the size. The plank is a little less than 3 and a half metres, and 345 cm is a sensible length for a wooden plank.
Answer: 345 cm, which is 3,450 mm.
Remember
- Ten millimetres make one centimetre.
- One hundred centimetres make one metre, and a metre is 1,000 mm.
- One kilometre is 1,000 metres.
- Choose the unit that suits the size of the object.
Try it
Answer the five questions below. Work them out first, then open each answer to check.
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How many millimetres are in one centimetre?
Show answer
10 millimetres. Ten small millimetre marks make one centimetre.
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How many centimetres are in two metres?
Show answer
200 centimetres. Each metre has 100 centimetres, and 2 × 100 = 200 cm.
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A pencil is 14 cm long. Write its length in millimetres.
Show answer
140 millimetres. 14 × 10 = 140 mm.
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Which is longer, 2 metres or 250 centimetres?
Show answer
250 centimetres. 2 m is 200 cm, so 250 cm is longer by 50 cm.
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A path is 3 km long. How many metres is that?
Show answer
3,000 metres. Each kilometre is 1,000 m, and 3 × 1,000 = 3,000 m.
Puzzle · Ruler marks
Puzzle time
A pencil lies on a 15 cm ruler. It starts at the 2 cm mark and is 9 cm long. At which mark does the pencil end?
Show a hint
Add the length of the pencil to the mark where it starts: 2 + 9.
Show the answer
Answer: 11 cm. The pencil starts at 2 cm. Nine more centimetres along the ruler is 11 cm, so the pencil ends at the 11 cm mark.