Converting millimetres and centimetres

Quick answer

There are 10 millimetres (mm) in 1 centimetre (cm). To convert millimetres to centimetres, divide by 10. To convert centimetres to millimetres, multiply by 10.

Learning outcome

By the end of this page, you will be able to convert between millimetres and centimetres using the fact that 10 mm = 1 cm.

Success signs:

• You can change 20 mm into cm and get 2 cm
• You can change 5 cm into mm and get 50 mm
• You can explain why 10 mm = 1 cm
• You can use millimetres and centimetres to measure things around you

Why this matters

Knowing how to convert between millimetres and centimetres helps you measure things more accurately. It's useful when you're building with LEGO, drawing pictures, cooking, or even when you're helping your parent buy fabric or paint.

Many rulers have both millimetres and centimetres marked on them. If you can convert between them, you can read any measurement on your ruler. This skill will help you in maths at school and in real life too!

How it works

Let's think about what we already know:

1. 1 centimetre is the same as 10 millimetres
2. This means 1 cm = 10 mm
3. If we want to go from millimetres to centimetres, we divide by 10
4. If we want to go from centimetres to millimetres, we multiply by 10

Think of it like this: if you have 10 small LEGO bricks and you want to make 1 big brick, you have 10 small ones that make 1 big one. The same is true with millimetres and centimetres!

Examples you can see

Let's look at some examples using objects you can imagine or touch:

Example 1: Converting millimetres to centimetres

You have 30 millimetres. How many centimetres is that?

30 ÷ 10 = 3

So 30 mm = 3 cm

Example 2: Converting centimetres to millimetres

You have 7 centimetres. How many millimetres is that?

7 × 10 = 70

So 7 cm = 70 mm

Example 3: Real-world measurement

Imagine you're making a picture frame. The width is 15 cm. How many millimetres wide is it?

15 × 10 = 150

So the frame is 150 mm wide

Example 4: Using a ruler

If you look at your ruler, you'll see that 10 small lines make 1 big centimetre. Each of those small lines is 1 millimetre.

So 10 mm = 1 cm

Try it as a mini adventure

Adventure: The Ruler Race!

You're going on a treasure hunt! You need to follow clues that tell you how far to walk in millimetres or centimetres.

1st clue: Walk 50 mm forward

2nd clue: Turn right and walk 3 cm forward

3rd clue: Walk 70 mm forward

4th clue: Turn left and walk 2 cm forward

Let's work out how far you walk in total:

1. 50 mm = 5 cm (50 ÷ 10 = 5)
2. 3 cm stays as 3 cm
3. 70 mm = 7 cm (70 ÷ 10 = 7)
4. 2 cm stays as 2 cm

Total distance: 5 + 3 + 7 + 2 = 17 cm

You walked 17 centimetres in total!

Common mistakes

• Mistake: Thinking 10 mm = 10 cm instead of 1 cm
• Why it happens: Confusing the unit names
• Fix: Remember that 10 mm = 1 cm. If you have 10 mm, you have 1 cm

• Mistake: Multiplying instead of dividing when going from mm to cm
• Why it happens: Mixing up which operation to use
• Fix: When going from small units (mm) to big units (cm), divide by 10

• Mistake: Adding instead of multiplying when going from cm to mm
• Why it happens: Not remembering the conversion rule
• Fix: When going from big units (cm) to small units (mm), multiply by 10

Parent and carer guide

Here's how you can help your child practice converting millimetres and centimetres:

1. Use a ruler at home. Ask your child to measure different items and convert the measurements
2. Play the ruler race game together
3. Look for millimetre and centimetre measurements in everyday life - like the width of a book, the length of a pencil, or the height of a plant
4. Use a measuring tape to measure things around your house
5. Practice the conversion table together

What to say:

• "Let's measure how tall you are in centimetres and then convert it to millimetres"
• "If we have 100 mm, how many cm is that?"
• "Can you find 3 things that are exactly 5 cm long?"

What not to worry about:

• Your child might make a mistake at first - that's normal
• Don't rush them. Let them think through the conversion
• It's okay if they need to count on their fingers or use a ruler to check

Practice questions

1. Convert 40 mm to cm
2. Convert 6 cm to mm
3. Convert 80 mm to cm
4. Convert 9 cm to mm
5. Convert 150 mm to cm
6. Convert 12 cm to mm

Answers:

1. 40 mm = 4 cm
2. 6 cm = 60 mm
3. 80 mm = 8 cm
4. 9 cm = 90 mm
5. 150 mm = 15 cm
6. 12 cm = 120 mm

FAQ

How many millimetres are in 1 centimetre?

There are 10 millimetres in 1 centimetre.

How do I know whether to multiply or divide?

If you're going from millimetres to centimetres, divide by 10. If you're going from centimetres to millimetres, multiply by 10.

Why is it useful to know this?

It helps you read rulers more easily, measure things more accurately, and understand how different units of measurement relate to each other.

Can I use this for other measurements?

Yes! This pattern works for other metric units too. For example, 100 centimetres = 1 metre, and 1000 metres = 1 kilometre.

What if I have a decimal number?

You still follow the same rule. For example, 15 mm = 1.5 cm (15 ÷ 10 = 1.5).