Grade by Grade
E. Spatial Sense · 3D Objects, Directions and Measurement

Millimetres, Centimetres, Metres and Kilometres

E2.2 E2.5About 35 minutes
Learning goals
  • I can explain how millimetres, centimetres, metres and kilometres are related.
  • I can measure small lengths in millimetres.
  • I can use benchmarks to estimate lengths.
  • I can choose a good unit for a length.
  • I can explain why a smaller unit gives a bigger count for the same length.

Words to know

millimetre (mm)
A very small unit of length. 10 millimetres make 1 centimetre. A dime is about 1 mm thick.
centimetre (cm)
A small unit of length. 100 centimetres make 1 metre. Your smallest fingernail is about 1 cm wide.
metre (m)
A unit for longer lengths. 1000 metres make 1 kilometre.
kilometre (km)
A unit for long distances, like the distance between towns. 1 km = 1000 m.
benchmark
Something you know well that helps you estimate, like "a fingernail is about 1 cm wide".
metric system
The system of units we use in Canada, like millimetres, centimetres, metres and kilometres.

Remember from Grade 2

In Grade 2 you measured with centimetres (cm) and metres (m), and learned that 100 cm = 1 m.

This year you meet two more units: the tiny millimetre (mm) and the long kilometre (km). Together they are the metric units of length we use in Canada.

Let's start with an ant and a road trip

How long is an ant? Less than 1 cm! We need a smaller unit.

How far is it to Grandma's house in another town? Many thousands of metres! We need a bigger unit.

That's why we have different units for different jobs.

Millimetres

Look closely at a ruler. Between each centimetre number there are 10 tiny spaces. Each tiny space is 1 millimetre.

10 mm = 1 cm

The little line in the middle (a bit longer) is at 5 mm, halfway to the next centimetre.

paper clip0123456small marks = millimetres, numbers = centimetres
A zoomed-in ruler. The paper clip goes from 0 to 4 small marks past 3 cm. That is 3 cm and 4 mm, or 34 mm.
Worked example

Example 1: Measure in millimetres

Look at the zoomed-in ruler. How long is the paper clip in millimetres?

  1. The paper clip starts at 0.
  2. Its end is past the 3, so it is more than 3 cm. 3 cm = 30 mm.
  3. Count the small marks past 3: there are 4 more millimetres.
  4. 30 + 4 = 34.

The paper clip is 34 mm long (3 cm and 4 mm).

Kilometres

For long distances, we use kilometres.

1000 m = 1 km

That is like 10 soccer fields lined up end to end, or walking for about 15 minutes at a normal speed. Road signs in Canada show distances in kilometres.

1 cm= 10 mm1 m= 100 cm1 km= 1000 mEach unit is made of smaller unitsmm → cm → m → km (smallest to biggest)
How the units are related: 10 mm make 1 cm, 100 cm make 1 m, and 1000 m make 1 km.
UnitBenchmark (about this long)Good for measuring
1 mmthe thickness of a dimean ant, a pencil tip, the thickness of a book cover
1 cmthe width of your smallest fingernaila crayon, a book, your hand
1 ma metre stick; a big step for a grown-upa bed, a room, a hallway
1 kmwalking for about 15 minutes; 2 and a half laps of a 400 m running trackthe distance to another neighbourhood or town
Benchmarks help you estimate lengths without a ruler.
Worked example

Example 2: Choose the unit

Which unit would you use to measure each thing: mm, cm, m or km?

  • the thickness of a cracker
  • the length of a pencil
  • the length of a hockey rink
  • the distance from Toronto to Ottawa
  1. A cracker is very thin, thinner than 1 cm: mm.
  2. A pencil is about as long as a hand: cm.
  3. A hockey rink is much longer than a room: m.
  4. Two cities are very far apart: km.

mm, cm, m, km.

Same length, different units

A pencil is 9 cm long. It is also 90 mm long. Did the pencil change? No!

Millimetres are smaller units, so you need more of them to cover the same length. Bigger units need fewer.

  • 1 cm = 10 mm, so 9 cm = 90 mm.
  • 1 m = 100 cm, so 2 m = 200 cm.
pencil01234567899 cm = 90 mm
The same pencil measured two ways: 9 cm or 90 mm. The pencil's length stays the same.
Worked example

Example 3: Changing units

Fill in the blanks.

  • 6 cm = ___ mm
  • 50 mm = ___ cm
  • 3 m = ___ cm
  • 2 km = ___ m
  1. Each cm is 10 mm. 6 groups of 10 = 60 mm.
  2. 50 mm is 5 groups of 10 mm = 5 cm.
  3. Each m is 100 cm. 100 + 100 + 100 = 300 cm.
  4. Each km is 1000 m. 1000 + 1000 = 2000 m.

6 cm = 60 mm, 50 mm = 5 cm, 3 m = 300 cm, 2 km = 2000 m.

Worked example

Example 4: Estimate with a benchmark

Estimate the height of a classroom door.

  1. Use the metre benchmark: a metre stick.
  2. Picture metre sticks stacked up the door. One metre stick reaches about to the doorknob. Two would reach about the top.
  3. So the door is about 2 metre sticks tall.

The door is about 2 m tall.

Say the unit!

A number alone doesn't tell a length. "The table is 80" could mean 80 mm (tiny) or 80 m (huge)! Always say the unit: "80 cm".

Just for fun

A tall giraffe can be about 5 metres high. That is 500 centimetres, or 5000 millimetres! Same giraffe, very different numbers.

Hands-on activity

Benchmark hunt

You need: a ruler with millimetres, a metre stick or measuring tape, and paper.

  1. Find something about 1 mm thick (a dime, a credit card, a fingernail). Check with the ruler.
  2. Find a part of your body about 1 cm wide. Check it.
  3. Find something about 1 m long. Check with the metre stick.
  4. Measure 3 small things in mm (a grain of rice, an eraser, a button). Then say each one in cm and mm too.
  5. With a grown-up, walk 1 km. You could use a 400 m school track (2 and a half laps) or a map app. How long did it take?

Grown-up note: stay with your child on the walk. Keep small items like buttons and rice away from babies.

Math talk: "Why do we need both millimetres and kilometres?"

Your turn

1

How many millimetres are in 1 centimetre?

2

How many centimetres are in 1 metre?

3

How many metres are in 1 kilometre?

4

Which unit would you use to measure the length of an ant?

5

Which unit would you use for the distance between two cities?

6

4 cm = ___ mm

7

A crayon is 8 cm long. How many millimetres is that?

More practice: warm-up

1

Which is the smallest unit: mm, cm, m or km?

2

1 m = ___ cm

3

Name a benchmark for 1 cm.

4

Would you measure the thickness of a coin in mm or m?

5

Would you measure the length of a swimming pool in cm or m?

More practice: on your own

1

60 mm = ___ cm

2

300 cm = ___ m

3

Put in order from shortest to longest: 1 km, 1 cm, 1 m, 1 mm.

4

Leah's bean plant is 5 cm and 3 mm tall. How many millimetres is that?

5

Which is longer: 45 mm or 5 cm? Explain.

Challenge

1

Mia walked 1500 m. Is that more or less than 1 km? By how much?

2

A ribbon is 1 m long. How many millimetres long is it? Explain.

3

A grown-up's big step is about 1 m. About how many big steps would it take to walk 1 km? About how many to walk 2 km?

Check your understanding

Quiz

1.

10 mm = ...

2.

1 km = ...

3.

Which unit is best for the distance between two towns?

4.

A dime is about how thick?

5.

A shoe is 20 cm long. How long is it in mm?

6.

Why does a pencil measure 90 mm but only 9 cm?

For parents and teachers

Big idea: Children learn how millimetres, centimetres, metres and kilometres are related (10 mm = 1 cm, 100 cm = 1 m, 1000 m = 1 km) and use benchmarks to estimate (Ontario E2.2). They also see that measuring the same length with a smaller unit gives a larger count, while the length itself stays the same (E2.5).

Common difficulties:

  • Mixing up which way to change. Ask: "Are millimetres bigger or smaller than centimetres? So will the number be bigger or smaller?"
  • Reading millimetre marks. Show that the middle, slightly longer mark is 5 mm.
  • Forgetting units. Ask "80 what?" whenever a unit is missing.
  • Picturing a kilometre. Walking it together makes it real.

Safety: supervise walks, and keep small items like buttons and rice away from babies and toddlers.

How to help: Use benchmarks out loud: "That puddle is about 2 m across", "The store is about 1 km away."