Chapter 1 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

M2.6.1 Chapter 1

Centimetres and metres

My table is 2 m long. Is that the same as 200 cm?

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 and draw lines to the nearest centimetre
  • Measure a room in metres and centimetres
  • Change 2 m 35 cm into 235 cm

Look at a ruler. It has a row of marks and numbers.

The gap between two numbers is one centimetre. We write cm.

To measure a pencil, line up one end with the 0 mark.

Draw it first

Measure a pencil

  1. A ruler marked from 0 to 10 cm. Step 2 lays a pencil along it, with one end on the 0 mark. Step 3 says: The pencil is 7 cm long.012345678910cm

    1 A ruler starts at 0. Find the 0 mark.

  2. A ruler marked from 0 to 10 cm. Step 2 lays a pencil along it, with one end on the 0 mark. Step 3 says: The pencil is 7 cm long.012345678910cm

    2 Put one end of the pencil on the 0 mark.

  3. A ruler marked from 0 to 10 cm. Step 2 lays a pencil along it, with one end on the 0 mark. Step 3 says: The pencil is 7 cm long.012345678910cmThe pencil is 7 cm long

    3 Read the number at the other end. It is 7 cm.

Do not start at the end of the ruler. Start at the 0 mark.

Sometimes the end falls between two marks.

The length to the nearest centimetre is the closest whole number.

Two sticks on two rulers. The top stick ends a little after the 6 mark, before the half way tick, so it is 6 cm. The bottom stick ends after the half way tick between 6 and 7, so it is 7 cm. The labels are 6 cm and 7 cm.012345678910cm6 cm012345678910cm7 cm
The short tick is half way between two numbers. Before it, stay on the lower number. Past it, go up.

A ruler is too short for a room. For long things we use the metre.

100 cm make 1 m. We write m.

Worked example

A table is 2 m 35 cm long. Write this in centimetres.

  1. 1 m is 100 cm. So 2 m is 200 cm.
  2. 2 m 35 cm is 200 cm and 35 cm more.
  3. 200 + 35 = 235.

Answer: 235 cm

See it three ways

With things

Lay a 1 m string along a wall two times. Then measure 35 cm more with a ruler.

As a picture

Three boxes in a row. Two long boxes are labelled 1 m and 1 m. One short box is labelled 35 cm. A line under all three is labelled 2 m 35 cm = 235 cm.1 m1 m35 cm2 m 35 cm = 235 cm

In symbols

200 cm + 35 cm = 235 cm.

Try it

Make a metre string and measure a room

You need: a piece of string, longer than 1 m, a ruler, a pencil.

  1. Put one end of the string on the 0 mark.
  2. Pinch the string at the ruler's last number, such as 30. Put the 0 mark on your pinch and repeat.
  3. Keep adding the centimetres. Stop when you reach 100 cm. Tie a knot there.
  4. Lay your string along one wall of a room. Count how many strings fit.
  5. Measure what is left with the ruler. Write the length as metres and centimetres.

What you will see: Your string is 1 m, which is 100 cm. You can change your answer to centimetres: each string is 100 cm.

If you cannot try it, this is what happens.
A wall measured with a metre string. Three boxes labelled 1 m, 1 m and 1 m, then a short box labelled 40 cm. A line under them is labelled 3 m 40 cm = 340 cm.1 m1 m1 m40 cm3 m 40 cm = 340 cm

A wrong idea many people have

200 cm is longer than 2 m. After all, 200 is more than 2.

Why it feels right: We learn that a bigger number means a bigger amount.

The test that breaks it: Measure a wall 3 m long two ways. Use your 1 m string, then a 30 cm ruler. The string fits 3 times. The ruler fits 10 times.

What is true: The wall did not change. A small unit fits more times, so its number is bigger. 200 cm and 2 m are the same length.

Key words

centimetre
a small length. The gap between two numbers on a ruler. We write cm.
metre
a length of 100 cm. We write m.
nearest
closest. The nearest centimetre is the closest whole number of cm.

Back to your guess

The answer is C: Yes. 100 cm make 1 m, so 2 m is 200 cm.

Yes. 100 cm make 1 m, so 2 m is 200 cm. The same table, two ways to say its length.

Check yourself

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

  1. How long is the stick?

    A stick lies on a ruler marked from 0 to 10 cm. One end is on the 0 mark. The other end is on the 8 mark.012345678910cm
    Tap your answer.

    1. Not this one. You may have counted every mark, including 0. The 0 mark is the start, not a centimetre. Try another.

    2. Yes, that is right. The stick starts at 0 and ends at the 8 mark. So it is 8 cm long.

    3. Not this one. You read the end of the ruler. Read the number where the stick ends. Try another.

    See the answer

    8 cm

    The stick starts at 0 and ends at the 8 mark. So it is 8 cm long.

    • If you chose A: You may have counted every mark, including 0. The 0 mark is the start, not a centimetre.
    • If you chose C: You read the end of the ruler. Read the number where the stick ends.

  2. A leaf is longer than 9 cm. Its tip is past the half way tick to 10. How long is it to the nearest centimetre?

    Tap your answer.

    1. Not this one. The tip is past half way. It is nearer to 10 than to 9. Go up to 10. Try another.

    2. Yes, that is right. The tip is past the half way tick. It is closer to 10 than to 9.

    See the answer

    10 cm

    The tip is past the half way tick. It is closer to 10 than to 9.

    • If you chose A: The tip is past half way. It is nearer to 10 than to 9. Go up to 10.

  3. Amina measures a wall. It is 4 m 5 cm long. How many centimetres is that?

    Tap your answer.

    1. Not this one. You wrote 5 cm as 50 cm. 5 cm is only 5. 4 m is 400 cm, and 400 + 5 = 405. Try another.

    2. Not this one. You joined 4 and 5. 4 m is 400 cm, not 4 cm. Try another.

    3. Yes, that is right. 4 m is 400 cm. 400 + 5 = 405.

    See the answer

    405 cm

    4 m is 400 cm. 400 + 5 = 405.

    • If you chose A: You wrote 5 cm as 50 cm. 5 cm is only 5. 4 m is 400 cm, and 400 + 5 = 405.
    • If you chose B: You joined 4 and 5. 4 m is 400 cm, not 4 cm.

  4. Change 3 m 20 cm into centimetres.

    See the answer

    320 cm

    3 m is 300 cm. 300 + 20 = 320.

  5. Which is longer, 150 cm or 2 m?

    Tap your answer.

    1. Not this one. You compared 150 with 2. Change to the same unit first. 2 m is 200 cm. Try another.

    2. Yes, that is right. 2 m is 200 cm. 200 is more than 150.

    3. Not this one. 2 m is 200 cm. 200 is not the same as 150. Try another.

    See the answer

    2 m

    2 m is 200 cm. 200 is more than 150.

    • If you chose A: You compared 150 with 2. Change to the same unit first. 2 m is 200 cm.
    • If you chose C: 2 m is 200 cm. 200 is not the same as 150.

  6. Draw a line 8 cm long with a ruler. How do you do it?

    See the answer

    Put a dot at the 0 mark. Put another dot at the 8 mark. Join the dots with the ruler as a guide.

    The line starts at 0 and ends at 8. So it is 8 cm long. Measure it again to check.

  7. A stick lies on a ruler. Its left end is on the 2 mark. Its right end is on the 9 mark. How long is the stick?

    A stick lies on a ruler marked from 0 to 10 cm. Its left end is above the 2 mark. Its right end is above the 9 mark.012345678910cm
    Tap your answer.

    1. Yes, that is right. The stick starts at 2 and ends at 9. The length is the gap between them. 9 − 2 = 7. You can also count the centimetres from 2 to 9: 7.

    2. Not this one. You read the right end only, as if it began at 0. The stick began at 2. The first 2 cm are not part of it. Try another.

    3. Not this one. You counted the numbers 2, 3, 4 and on to 9. That is 8 numbers. A length counts the centimetres between them. There are 7. Try another.

    See the answer

    7 cm

    The stick starts at 2 and ends at 9. The length is the gap between them. 9 − 2 = 7. You can also count the centimetres from 2 to 9: 7.

    • If you chose B: You read the right end only, as if it began at 0. The stick began at 2. The first 2 cm are not part of it.
    • If you chose C: You counted the numbers 2, 3, 4 and on to 9. That is 8 numbers. A length counts the centimetres between them. There are 7.

  8. A rope is 3 m long. How many centimetres is that?

    Tap your answer.

    1. Not this one. You used 10 cm for 1 m. A metre is 100 cm. So 3 m is 3 lots of 100 cm. Try another.

    2. Not this one. That is 1 000 cm for each metre. A metre is 100 cm. So 3 m is 300 cm. Try another.

    3. Yes, that is right. 1 m is 100 cm. 3 m is 3 × 100 = 300 cm.

    See the answer

    300 cm

    1 m is 100 cm. 3 m is 3 × 100 = 300 cm.

    • If you chose A: You used 10 cm for 1 m. A metre is 100 cm. So 3 m is 3 lots of 100 cm.
    • If you chose B: That is 1 000 cm for each metre. A metre is 100 cm. So 3 m is 300 cm.

Keep it

Copy this into your exercise book.

Put the 0 mark at one end. Read the other end. 100 cm make 1 m. 2 m 35 cm is 200 + 35 = 235 cm.

Draw: A ruler with a pencil from 0 to 7. Under it, write 100 cm = 1 m.

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.