Chapter 2 of 8

All 8 chapters
  1. 1Centimetres and metres, readunderstood
  2. 2Millimetres and kilometres, readunderstood
  3. 3Tenths and the decimal point, readunderstood
  4. 4Grams and kilograms, readunderstood
  5. 5Millilitres and litres, readunderstood
  6. 6Telling the time, readunderstood
  7. 7How long it takes, readunderstood
  8. 8Perimeter and area, readunderstood

M2.6.2 Chapter 2

Millimetres and kilometres

Which unit is for an ant? Which is for a long road?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Measure small things to the nearest millimetre
  • Say that 1 000 m make 1 km
  • Choose the right unit: mm, cm, m or km

A centimetre is too big for a grain of rice.

A metre is too small for the road to the next town.

A millimetre is very small. We write mm. Ten sheets of paper in a pile are about 1 mm thick.

Draw it first

Look closely at one centimetre

  1. A ruler from 0 to 5 cm. Step 2 adds the small marks between the numbers. Step 3 shows the first centimetre with a line under it, labelled 10 mm = 1 cm.012345cm

    1 Look at the gap from 0 to 1 on the ruler. It is 1 cm.

  2. A ruler from 0 to 5 cm. Step 2 adds the small marks between the numbers. Step 3 shows the first centimetre with a line under it, labelled 10 mm = 1 cm.012345cm

    2 Between 0 and 1 there are 10 small gaps. Each one is 1 mm.

  3. A ruler from 0 to 5 cm. Step 2 adds the small marks between the numbers. Step 3 shows the first centimetre with a line under it, labelled 10 mm = 1 cm.012345cm10 mm = 1 cm

    3 Count the small gaps. 10 mm make 1 cm.

10 mm make 1 cm. 100 cm make 1 m. So 1 000 mm make 1 m.

To read mm, first read the whole cm. Then count the small marks after it.

If the end is between two small marks, pick the nearer one.

A paper strip lies above a ruler from 0 to 5 cm. Dashed lines drop from its two ends. It starts at 0 and ends at 2 cm and 3 small marks.012345cm
The strip ends 2 cm and 3 small marks from 0.

Worked example

A paper strip is 2 cm 3 mm long. Write this in mm.

  1. 1 cm is 10 mm. So 2 cm is 20 mm.
  2. 2 cm 3 mm is 20 mm and 3 mm more.
  3. 20 + 3 = 23.

Answer: 23 mm

For a long way we use the kilometre. We write km.

See it three ways

With things

Walk along a field that is 100 m long. Walk its length 10 times. You have walked 1 000 m, or 1 km.

As a picture

A bar cut into 10 equal parts. Each part is 100 m. The label under it says 10 × 100 m = 1 km.10 × 100 m = 1 km

In symbols

10 × 100 m = 1 000 m = 1 km.

Most people walk 1 km in about 12 to 15 minutes.

Pick the unit that fits the thing you measure.

A table with two columns, Unit and Use it for. mm is for an ant. cm is for a book. m is for a room. km is for a long road.UnitUse it formman antcma bookma roomkma long road
A good unit gives a number you can picture.

Try it

Measure small things in mm

You need: a ruler with mm marks, a bean, a seed or a button.

  1. Lay the small thing next to the ruler. Put one end on the 0 mark.
  2. Read the whole cm just before the other end.
  3. Count the small marks after it. Each one is 1 mm.
  4. Write the length in mm. A length of 1 cm 4 mm is 14 mm.

What you will see: You count small marks of 1 mm each. A grain of rice is only a few mm wide.

If you cannot try it, this is what happens.
A bean drawn as a hatched strip above a ruler from 0 to 5 cm. It ends at 1 cm and 4 small marks. The label says 1 cm 4 mm = 14 mm.012345cm1 cm 4 mm = 14 mm

A wrong idea many people have

1 km is 100 m, because 100 cm make 1 m.

Why it feels right: Going from cm to m uses 100. It seems that m to km will too.

The test that breaks it: Walk across a field 100 m long. It takes a bit more than a minute. Walking 1 km takes 12 to 15 minutes. That is about 10 times as long.

What is true: 1 km is 1 000 m. It is 10 fields of 100 m, end to end.

Key words

millimetre
a very small length. 10 mm make 1 cm. We write mm.
kilometre
a long length. 1 000 m make 1 km. We write km.

Back to your guess

The answer is B: Use mm for the ant and km for the road.

Use a small unit for a small thing. Use a big unit for a long way. So mm for the ant, and km for the road.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Which unit is best for the distance between two towns?

    Tap your answer.

    1. Not this one. A millimetre is tiny. It is for things like a grain of rice. Try another.

    2. Not this one. A metre suits a room. Towns are thousands of metres apart. Use km. Try another.

    3. Yes, that is right. Towns are far apart. A kilometre is a long length, so it fits.

    See the answer

    km

    Towns are far apart. A kilometre is a long length, so it fits.

    • If you chose A: A millimetre is tiny. It is for things like a grain of rice.
    • If you chose B: A metre suits a room. Towns are thousands of metres apart. Use km.

  2. The strip ends at 3 cm and 7 small marks. How many mm long is it?

    A paper strip lies above a ruler from 0 to 5 cm. It starts at 0 and ends at 3 cm and 7 small marks.012345cm
    See the answer

    37 mm

    3 cm is 30 mm. Then 7 small marks make 7 mm more. 30 + 7 = 37.

  3. Change 4 km into metres.

    See the answer

    4 000 m

    1 km is 1 000 m. So 4 km is 4 × 1 000 = 4 000 m.

  4. From an earlier chapter

    Kofi has a string 1 m 40 cm long. How many centimetres is that?

    See the answer

    140 cm

    1 m is 100 cm. 100 + 40 = 140.

  5. A road is 2 km 500 m long. How many metres is that?

    See the answer

    2 500 m

    2 km is 2 000 m. 2 000 + 500 = 2 500.

  6. How many mm make 1 m?

    See the answer

    1 000 mm

    1 m is 100 cm. Each cm is 10 mm. So 100 × 10 = 1 000.

  7. A seed ends 1 cm and 2 small marks from 0. How many mm long is it?

    A seed lies above a ruler from 0 to 5 cm. It starts at 0 and ends at 1 cm and 2 small marks.012345cm
    Tap your answer.

    1. Not this one. You read 1 and 2 together as 12 cm. The seed is only 1 cm and 2 mm long. Try another.

    2. Yes, that is right. 1 cm is 10 mm. Then 2 small marks make 2 mm more. 10 + 2 = 12.

    3. Not this one. 1 cm is 10 mm, not 100 mm. So 1 cm 2 mm is 10 + 2 = 12 mm. Try another.

    See the answer

    12 mm

    1 cm is 10 mm. Then 2 small marks make 2 mm more. 10 + 2 = 12.

    • If you chose A: You read 1 and 2 together as 12 cm. The seed is only 1 cm and 2 mm long.
    • If you chose C: 1 cm is 10 mm, not 100 mm. So 1 cm 2 mm is 10 + 2 = 12 mm.

  8. Which is the longest: 900 m, 1 500 mm or 1 km?

    Tap your answer.

    1. Not this one. You may have used 100 m for 1 km. 1 km is 1 000 m. That is more than 900 m. Try another.

    2. Not this one. You picked the biggest number. 1 500 mm is 1 m 50 cm, which is short. Try another.

    3. Yes, that is right. Use metres for all three. 1 000 mm is 1 m. So 1 500 mm is less than 2 m. 1 km is 1 000 m. The most is 1 km.

    See the answer

    1 km

    Use metres for all three. 1 000 mm is 1 m. So 1 500 mm is less than 2 m. 1 km is 1 000 m. The most is 1 km.

    • If you chose A: You may have used 100 m for 1 km. 1 km is 1 000 m. That is more than 900 m.
    • If you chose B: You picked the biggest number. 1 500 mm is 1 m 50 cm, which is short.

  9. Ravi has a pencil 14 cm long. How many mm is that?

    Tap your answer.

    1. Yes, that is right. 1 cm is 10 mm. So 14 cm is 14 × 10 = 140 mm.

    2. Not this one. You kept the same number. But 1 cm is 10 mm. So 14 cm is 14 lots of 10. Try another.

    3. Not this one. You used 100 mm for 1 cm. 1 cm is only 10 mm. So 14 cm is 140 mm. Try another.

    See the answer

    140 mm

    1 cm is 10 mm. So 14 cm is 14 × 10 = 140 mm.

    • If you chose B: You kept the same number. But 1 cm is 10 mm. So 14 cm is 14 lots of 10.
    • If you chose C: You used 100 mm for 1 cm. 1 cm is only 10 mm. So 14 cm is 140 mm.

  10. Which length is about right for a pencil?

    Tap your answer.

    1. Not this one. You may have mixed up mm and cm. 18 mm is less than 2 cm, about a thumb wide. Try another.

    2. Yes, that is right. A pencil is about as long as a hand. That is about 18 cm. 18 mm is too short and 18 m is too long.

    3. Not this one. You may have mixed up m and mm. 18 m is longer than most classrooms. A pencil fits in your hand. Try another.

    See the answer

    18 cm

    A pencil is about as long as a hand. That is about 18 cm. 18 mm is too short and 18 m is too long.

    • If you chose A: You may have mixed up mm and cm. 18 mm is less than 2 cm, about a thumb wide.
    • If you chose C: You may have mixed up m and mm. 18 m is longer than most classrooms. A pencil fits in your hand.

Keep it

Copy this into your exercise book.

10 mm make 1 cm. 1 000 mm make 1 m. 1 000 m make 1 km. Use mm for an ant. Use cm for a book. Use m for a room. Use km for a road.

Draw: A ruler with 1 cm split into 10 small gaps. Write 10 mm = 1 cm.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (882 KB). It opens on any phone.