Length — centimetre and metre

For measuring length we use two main units:

• Centimetre (cm) — for small things.
• Metre (m) — for big things.

Remember: 1 metre = 100 centimetres.

When centimetres

We use centimetres for things that fit on a ruler:

• Pen ≈ 14 cm
• Book (height) ≈ 25 cm
• Child's hand ≈ 15 cm

Try it: take a ruler and measure your finger. How long is it?

When metres

Metres are for bigger things:

• Table ≈ 1 m
• Door (height) ≈ 2 m
• Car (length) ≈ 4 m

Trick: if a thing can be hugged with your arms or walked across in steps, use metres. If it fits on a book, use centimetres.

Conversion: metre ↔ centimetre

Since 1 m = 100 cm, we can switch:

• 1 m = 100 cm
• 2 m = 200 cm
• 50 cm = half a metre
• 150 cm = 1 m + 50 cm = a metre and a half

How to measure with a ruler

1. Put the thing on the zero of the ruler (at the start).
2. See where it ends.
3. Read the number — that is the length in centimetres.

Common mistakes:

• Starting from 1 cm instead of 0 → the answer is 1 cm too big.
• The ruler is not in line with the thing → the answer is inaccurate.

When to estimate

Sometimes a rough guess is enough — without a ruler:

• "This room is about 3 metres." (about three steps)
• "The pen is about 15 cm." (palm to fingertip)

Estimating is useful — when you don't have a ruler at hand.

Summary

• Centimetre (cm) for small things, metre (m) for big.
• 1 m = 100 cm.
• The ruler measures from 0, not from 1.
• Estimates are useful — roughly, without a tool.