T

Length: Metres, Centimetres & Millimetres

Maths · Y3 Measurement
Year 3
Lesson Guide
Explanation
Length has three main units, sized for different jobs. Millimetres (mm) measure tiny things — an ant is about 4 mm. Centimetres (cm) measure hand-sized things — your fingernail is about 1 cm wide. Metres (m) measure big things — a door is about 2 m tall. The magic numbers: 10 mm make 1 cm, and 100 cm make 1 m.
Your Turn
Which unit for: a ladybird? your desk? the school hall? Check: mm, cm, m.
Watch Out
Don't measure a room in mm — you'd be counting to thousands! Match the unit to the size of the thing.
Explanation
Rulers have one golden rule: line up the END of your object with the ZERO mark — not with the edge of the ruler! Then read where the other end lands. The little marks between the numbered centimetres are millimetres — ten of them per centimetre — so a pencil ending 3 small marks past 5 is 5 cm 3 mm, or 53 mm.
Your Turn
Measure a real pencil or your finger with a ruler — carefully from the 0. Write it as cm AND mm.
Watch Out
Starting from the ruler edge (before the 0) makes everything too long. Zero first, always.
Explanation
Sometimes you need the same length in a different unit. Going from a big unit to a small one, multiply: 3 cm = 30 mm (×10), 2 m = 200 cm (×100). Going the other way, bundle up: 150 cm has one full hundred (1 m) with 50 cm left over — so 150 cm = 1 m 50 cm. That's exactly how we say people's heights!
Your Turn
Convert: 7 cm to mm, 4 m to cm, and 125 cm to m-and-cm. Check: 70 mm, 400 cm, 1 m 25 cm.
Watch Out
cm → mm uses ×10, but m → cm uses ×100. Pick the right magic number for the jump!
Explanation
Which is longer: 1 m of ribbon or 90 cm? You can't compare the bare numbers 1 and 90 — they're in different units! Convert first so units match: 1 m = 100 cm. Now compare fairly: 90 cm < 100 cm < 110 cm. Rule for every comparison: same units first, then compare the numbers.
Your Turn
Which is longer: 2 m or 150 cm? Check: 2 m = 200 cm > 150 cm — the 2 m one.
Watch Out
The number 90 LOOKS bigger than 1 — units first or the numbers lie to you!
Explanation
You carry a measuring kit everywhere: your fingernail is about 1 cm, your stretched hand about 15 cm, one big step about 1 m. Use them to ESTIMATE before you measure — 'the desk looks like 4 hand spans… about 60 cm' — then check with a ruler. Estimating first makes you notice when a measurement has gone wrong. Quiz and exercises now!
Your Turn
Estimate the length of your bed in big steps, then in metres. A bed is usually a bit under 2 m — were you close?
Watch Out
An estimate is a smart guess, not a wild one — always anchor it to a body ruler you know.
Step 1: Introduction
Step 1 of 5
mm, cm, m — pick the right tool 🐜 millimetre (mm) tiny things: an ant, a grain of rice 📏 centimetre (cm) hand-sized things: a pencil, a book 🚪 metre (m) big things: a door, a room, a pool 10 mm = 1 cm · 100 cm = 1 m Your fingernail is about 1 cm wide. A big step is about 1 m. Body rulers help you estimate without measuring!
Start at ZERO, not at the edge 0 1 2 3 4 5 6 7 8 pencil Line up the object's END with the 0 mark. Read where the OTHER end lands: this pencil is 5 cm. Small lines between numbers are millimetres — 10 per cm.
Swap between units 3 cm = 3 × 10 = 30 mm 2 m = 2 × 100 = 200 cm 150 cm = 1 m and 50 cm Big unit → small unit: MULTIPLY. Small → big: split into groups.
Which ribbon is longest? 1 m (= 100 cm) 90 cm 110 cm To compare, make the units MATCH first: 1 m = 100 cm. 90 cm < 100 cm < 110 cm — the 110 cm ribbon wins! Never compare 1 (m) with 90 (cm) as bare numbers — units first!
Your built-in measuring kit 💅 fingernail width ≈ 1 cm 🖐 hand span (stretched) ≈ 15 cm 👣 one big step ≈ 1 m 🚌 a school bus ≈ 12 m Estimate first, measure second — good estimators are hard to fool!
Was this lesson helpful? Rate this Course
Discussion
Loading comments...