X Tables
measurement

Converting units with decimals

Use place value to convert between metric units.

Quick answer

You can convert between metric units with decimals by using place value – moving the decimal point when going from larger units to smaller units, or moving it backwards when going from smaller to larger units.

Learning outcome

By the end of this page, you will be able to convert between metric units (like metres, centimetres, millimetres) using decimals and understand how place value helps you move the decimal point. Success signs:
  • You can convert 1.5 metres to centimetres and millimetres
  • You can explain why 2.3 kilometres is the same as 2300 metres
  • You can use a place value chart to help you convert units
  • You can solve real-life problems involving measurements with decimals

Why this matters

Converting units with decimals is useful in everyday life – like when you're cooking and need to measure ingredients, or when you're planning a trip and need to know how far you’ve walked or driven. It's also important in science, sports, and building things. Understanding how to move decimal points helps you solve problems quickly and accurately.

How it works

To convert between metric units with decimals, we use place value. The metric system is based on tens – each unit is ten times larger or smaller than the next one. Here’s how it works step by step:
  1. First, you need to know the order of metric units from largest to smallest: kilometre (km), hectometre (hm), decametre (dam), metre (m), decimetre (dm), centimetre (cm), millimetre (mm).
  2. When you move from a larger unit to a smaller unit, you move the decimal point to the right (add zeros or move the decimal point). For example, 1 metre = 100 centimetres.
  3. When you move from a smaller unit to a larger unit, you move the decimal point to the left (remove zeros or move the decimal point). For example, 100 centimetres = 1 metre.
  4. Use a place value chart to help you remember how far to move the decimal point.

Examples you can see

Let’s look at some examples to help you understand. Example 1: Converting metres to centimetres If you have 2.5 metres, how many centimetres is that? 1 metre = 100 centimetres So, 2.5 metres = 2.5 × 100 = 250 centimetres Example 2: Converting centimetres to metres If you have 350 centimetres, how many metres is that? 100 centimetres = 1 metre So, 350 centimetres = 350 ÷ 100 = 3.5 metres Example 3: Converting kilometres to metres If you have 4.2 kilometres, how many metres is that? 1 kilometre = 1000 metres So, 4.2 kilometres = 4.2 × 1000 = 4200 metres Example 4: Converting millimetres to centimetres If you have 75 millimetres, how many centimetres is that? 10 millimetres = 1 centimetre So, 75 millimetres = 75 ÷ 10 = 7.5 centimetres

Try it as a mini adventure

Imagine you are a space explorer on a mission to collect samples from different planets. Each planet has a different size measurement system, and you need to convert your findings into Earth units to compare them. Planet Zog’s sample: 3.2 kilometres of rock samples Planet Xylo’s sample: 4500 metres of soil samples Planet Yara’s sample: 750 millimetres of water samples Your task is to convert all the samples to the same unit – metres – so you can compare them.
  1. Planet Zog: 3.2 km = 3.2 × 1000 = 3200 metres
  2. Planet Xylo: 4500 metres = 4500 metres
  3. Planet Yara: 750 mm = 750 ÷ 10 = 75 centimetres = 0.75 metres
Now you can compare them easily! Which planet had the largest sample? Which had the smallest?

Common mistakes

  • Mistake: Forgetting to move the decimal point when converting units. Why it happens: Children may not fully understand that each step in the metric system is ten times larger or smaller. Fix: Use a place value chart or count how many steps you move between units to know how many places to move the decimal point.
  • Mistake: Adding zeros instead of moving the decimal point. Why it happens: Confusion between moving the decimal point and adding zeros. Fix: Remember: moving the decimal point to the right makes the number bigger, and moving it to the left makes it smaller.
  • Mistake: Not checking if the answer makes sense. Why it happens: Rushing through the problem without thinking. Fix: Always think: “Does this answer make sense?” For example, 1.5 km should be more than 1500 metres, not less.

Parent and carer guide

Parents and carers can help by:
  • Using everyday situations like cooking or shopping to practice conversions
  • Using a place value chart or a conversion chart to help visualise the process
  • Practising with real-life examples such as measuring a room or comparing the heights of family members
  • Encouraging children to explain their thinking and check their answers
  • Reinforcing that moving the decimal point is like “sliding” the number along the place value chart

Practice questions

  1. Convert 5.3 metres to centimetres.
  2. Convert 800 centimetres to metres.
  3. Convert 2.7 kilometres to metres.
  4. Convert 650 millimetres to centimetres.
  5. Convert 1.25 kilometres to metres.
Answers:
  1. 5.3 × 100 = 530 centimetres
  2. 800 ÷ 100 = 8 metres
  3. 2.7 × 1000 = 2700 metres
  4. 650 ÷ 10 = 65 centimetres
  5. 1.25 × 1000 = 1250 metres

FAQ

How do I know which way to move the decimal point?

If you're converting from a larger unit to a smaller one (like km to m), move the decimal point to the right. If you're converting from a smaller unit to a larger one (like cm to m), move the decimal point to the left.

Why is the metric system based on tens?

The metric system was designed to be simple and easy to use. Since our number system is based on tens, it makes sense to use tens in measurements too.

Can I use a calculator for unit conversions?

Yes, calculators are helpful for checking your work. But it’s important to understand the process first so you know if your answer is reasonable.

What if I’m not sure which unit is bigger or smaller?

You can use the order: km, hm, dam, m, dm, cm, mm. Remember that the further you go to the right, the smaller the unit.

Is it okay to have decimals in measurements?

Yes! In real life, many measurements are not whole numbers. Decimals help us be more accurate when measuring things like height, distance, or ingredients.