Units of Length: mm, cm, m and km

ME1Units of Length

mm, cm, m and km — and when to use each.

Length is measured in millimetres, centimetres, metres and kilometres.

Pick the sensible unit.

width of a fingernail ≈ 10 mm

width of this card ≈ 8 cm

height of a door ≈ 2 m

a 15-minute walk ≈ 1 km

10 mm = 1 cm · 100 cm = 1 m · 1000 m = 1 km

Learn the Units

1Small things → mm and cm.

Millimetres for tiny widths; centimetres for pencils, books, people's heights.

✏️ 18 cm

2Rooms and buildings → m.

Metres for doors, rooms, swimming pools, football pitches.

🚪 2 m

3Journeys → km.

Kilometres for distances between places.

🚗 12 km

💡 Key IdeaThe key facts: 10 mm = 1 cm · 100 cm = 1 m · 1000 m = 1 km. Everything in this topic builds on these three.

Worked Example

Choose the best unit for each measurement.

Length of an ant

Height of a friend

cm (or m)

Length of a bus

London to Leeds

Watch Out! Common Mistakes

Muddling the ladder.

“100 mm = 1 cm” ✕

Wrong! 10 mm = 1 cm. It’s 100 cm that make 1 m.

Silly units.

“My pencil is 18 m long” ✕

Sense-check! 18 m is longer than a bus. A pencil is 18 cm.

Missing units in answers.

Length = 25 ✕

25 what? A measurement without a unit is meaningless — and loses the mark. Write 25 cm.

⬇ Cheat Sheet

UNITS OF LENGTH

10 mm = 1 cm
100 cm = 1 m
1000 m = 1 km
mm: tiny · cm: hand-sized to person-sized · m: rooms · km: journeys

Example: ant → mm · friend → cm · bus → m · city to city → 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

→

Next Lesson: ME2 Converting Length× to go smaller units, ÷ to go bigger.

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

Quick summary

Millimetres for tiny things, centimetres for everyday objects, metres for rooms, kilometres for journeys.
The ladder: 10 mm = 1 cm, 100 cm = 1 m, 1000 m = 1 km.
Always sense-check: could a pencil really be 18 m long?
Never write a measurement without its unit.

Frequently asked questions

What are the metric units of length?

Millimetre (mm), centimetre (cm), metre (m) and kilometre (km). 10 mm = 1 cm, 100 cm = 1 m and 1000 m = 1 km.

How do children choose the right unit?

By anchoring to familiar objects: a fingernail is about 1 cm wide, a door about 2 m tall, and a 15-minute walk about 1 km.

When is this taught at school?

Choosing units and estimating lengths runs from Year 2, with conversions between all metric length units expected by Years 5–6.

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ME2 Converting Length