Converting Length: mm, cm, m, km (KS2)

ME2Converting Length

× to go smaller units, ÷ to go bigger.

Convert between mm, cm, m and km by multiplying or dividing by 10, 100 or 1000.

3.45 m = ? cm

m → cm: × 100

3.45 × 100 = 345 cm

Going to a SMALLER unit needs MORE of them — so multiply.

cm→mm ×10 · m→cm ×100 · km→m ×1000 — and divide to go back.

Learn the 3-Step Method

1Find the conversion number.

mm↔cm: 10. cm↔m: 100. m↔km: 1000.

10 · 100 · 1000

2Decide × or ÷.

To a smaller unit → multiply (more of them). To a bigger unit → divide.

smaller unit → ×

3Slide the digits.

×100 slides digits two places left; ÷100 two places right.

3.45 → 345

💡 Key IdeaThis is the decimals digit-sliding skill in disguise — metric units are built on 10, 100 and 1000.

Worked Example

Convert each measurement.

4.5 cm → mm

× 10

45 mm

345 cm → m

÷ 100

3.45 m

2.6 km → m

× 1000

2600 m

750 m → km

÷ 1000

0.75 km

Watch Out! Common Mistakes

Multiplying when you should divide.

750 m = 750 000 km ✕

Sense-check! A km is BIGGER than a metre, so you need fewer: 750 ÷ 1000 = 0.75 km.

Wrong conversion number.

3.45 m = 3.45 × 10 = 34.5 cm ✕

Wrong! m→cm uses 100: 3.45 × 100 = 345 cm.

Mixing units mid-calculation.

1.2 m + 30 cm = 31.2 ✕

Wrong! Convert first: 120 cm + 30 cm = 150 cm (or 1.5 m).

⬇ Cheat Sheet

CONVERTING LENGTH

mm↔cm: 10 · cm↔m: 100 · m↔km: 1000
To a smaller unit → multiply.
To a bigger unit → divide.
Convert everything to the same unit before adding.

Examples: 3.45 m = 345 cm · 2.6 km = 2600 m · 750 m = 0.75 km

📖 Continue Your Journey

✓ ME1 Units of Length ME2 Converting Length ME3 Units of Mass ME4 Converting Mass ME5 Units of Capacity ME6 Converting Capacity ME7 Reading Scales ME8 Perimeter ME9 Area of Rectangles ME10 Area of Triangles ME11 Area of Parallelograms ME12 Compound Shapes ME13 Volume of Cuboids ME14 Telling the Time ME15 Time Calculations ME16 Money Calculations ME17 Imperial and Metric Units ME18 Measurement Problem Solving

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Next Lesson: ME3 Units of MassGrams and kilograms — how heavy?

📄 Free printable: Download the Converting Length cheat sheet (PDF) — keep it by your child's desk.

Quick summary

Conversion numbers: 10 (mm↔cm), 100 (cm↔m), 1000 (m↔km).
Converting to a smaller unit → multiply; to a bigger unit → divide.
Slide digits — 3.45 m × 100 = 345 cm.
Before adding lengths, convert them to the same unit.

Frequently asked questions

How do you convert metres to centimetres?

Multiply by 100 — the digits slide two places left. 3.45 m = 345 cm.

How do you know whether to multiply or divide?

Think about size: a smaller unit means you need more of them (multiply); a bigger unit means fewer (divide). 750 m is less than 1 km, so 0.75 km.

Why do children find metric conversion easy after decimals?

Because it's the same skill — multiplying and dividing by 10, 100 and 1000 by sliding digits. Our Decimals lessons D9–D10 teach exactly that.