E. Spatial Sense · Shapes, Maps and Measurement

Centimetres and Metres

E2.2About 30 minutes

Learning goals

I can explain why we use standard units like centimetres and metres.
I can tell that 100 centimetres make 1 metre.
I can use benchmarks (like my fingertip) to estimate lengths.
I can choose centimetres or metres to measure something.

Words to know

standard unit
A unit that is the same everywhere, for everyone. Centimetres and metres are standard units.

centimetre (cm)
A small unit of length. Your fingertip is about 1 cm wide.

metre (m)
A bigger unit of length. 1 metre is the same as 100 centimetres.

benchmark
Something you know the size of, that helps you estimate. "My fingertip is about 1 cm wide."

Why do we need the same units?

In the last lesson, Zoe and her dad got different numbers for the same table, because their hands are different sizes.

To fix this, people everywhere agreed on standard units. In Canada we use centimetres and metres. A centimetre is the same length in Toronto, Iqaluit and Vancouver!

The centimetre

A centimetre is small. We write it cm.

Your fingertip is about 1 cm wide. Look at a ruler: the spaces between the numbers are centimetres.

A ruler shows centimetres. A fingertip is about 1 cm wide.

The metre

A metre is much longer. We write it m.

100 centimetres make 1 metre. That's a lot of fingertips!

A metre stick is exactly 1 metre long. A door handle is often about 1 metre up from the floor. A long, big step for a grown-up is about 1 metre.

A metre stick has 100 centimetres. Count by 10s: 10, 20, 30 ... 100.

Benchmark	About how long?
width of your fingertip	1 cm
width of a loonie	between 2 cm and 3 cm
a new crayon	about 9 cm
a sheet of paper (long side)	about 28 cm
a metre stick	exactly 1 m (100 cm)
from the floor to a door handle	about 1 m

Benchmarks help you estimate. Measure your own to check!

Worked example

Example 1: Centimetres or metres?

Which unit would you use to measure (a) a pencil, (b) the length of a hockey rink?

A pencil is short, about as long as a few fingertips in a row. A small unit works: centimetres.
A hockey rink is very long. Using centimetres would give a huge number. A big unit works: metres.

(a) centimetres (b) metres

Worked example

Example 2: Estimate with a benchmark

About how long is an eraser? Use your fingertip as a benchmark.

Put your fingertip at one end of the eraser.
Move it along, fingertip after fingertip, and count: 1, 2, 3, 4, 5.
It took about 5 fingertips. Each fingertip is about 1 cm.

The eraser is about 5 cm long. (Your eraser may be a little different. That's fine for an estimate!)

Remember the big-and-small rule

A metre is a bigger unit than a centimetre. So the same thing has a bigger number of centimetres than of metres. A doorway might be 2 m tall, which is the same as 200 cm.

Worked example

Example 3: How many centimetres?

A ribbon is 2 metres long. How many centimetres is that?

1 m = 100 cm.
2 m is 100 cm + 100 cm.
100 + 100 = 200.

200 cm.

Just for fun

A tall giraffe can be about 5 metres tall. That's 500 centimetres, or about 500 fingertips stacked on top of each other. Good luck measuring it with a ruler!

Hands-on activity

Make a metre

You need: string or yarn, scissors, a ruler or measuring tape (if you have one), tape.

With a grown-up, cut a piece of string 1 metre long (100 cm). If you don't have a measuring tool, use a sheet of paper's long side: about 28 cm. 1 metre is a bit shorter than 4 sheets laid end to end.
Find 3 things that are longer than 1 m, 3 things shorter than 1 m, and 1 thing about 1 m.
Stand beside your string. Is your height more or less than 1 m?
Find your own 1 cm benchmark: which finger is about 1 cm wide?

Math talk: "Should we measure the couch in centimetres or metres? Why?"

Your turn

How many centimetres are in 1 metre?

Which is longer: 1 cm or 1 m?

What body part is about 1 cm wide?

Would you measure a ladybug in centimetres or metres?

Would you measure the length of a swimming pool in centimetres or metres?

What does cm stand for? What does m stand for?

Is a classroom door taller or shorter than 1 metre?

More practice: warm-up

Fill in the blank: 1 m = ___ cm

Which unit is smaller: centimetre or metre?

Which unit would you use to measure a paper clip?

Which unit would you use to measure a soccer field?

What is a benchmark?

More practice: on your own

A bed is about 2 m long. About how many centimetres is that?

Jay says his pencil is 15 m long. What mistake did he make?

Order from shortest to longest: 1 m, 30 cm, 2 m.

A new crayon is about 9 cm long. About how long are 2 crayons end to end?

Which is longer: 90 cm or 1 m? How do you know?

Challenge

Amira's skipping rope is 3 m long. How many centimetres is that?

A plank is 50 cm long. How many planks laid end to end make 1 metre?

A paper is about 28 cm long. Is a row of 4 sheets end to end more or less than 1 metre? Explain.

Check your understanding

Quiz

How many centimetres are in 1 metre?

Which is a good benchmark for 1 cm?

Which unit would you use to measure the length of a school hallway?

Which unit would you use to measure a crayon?

Which length is the longest?

Why do we use standard units like centimetres?

For parents and teachers

Big idea: Standard units let everyone get the same measurement. 100 cm = 1 m. Children should develop personal benchmarks (fingertip ≈ 1 cm, door handle height ≈ 1 m) to make sensible estimates and choose units.

Common difficulties:

Choosing the wrong unit ("my pencil is 15 m"). Hold up a metre stick or string and ask, "Is your pencil this long?"
Thinking a bigger number always means longer, without checking units (thinking 90 cm is longer than 1 m). Compare using a metre string.
Benchmarks vary. A child's fingertip may be a little more or less than 1 cm. That's fine for estimating.

How to help: Make a 1-metre string and keep it handy for "longer than / shorter than a metre" hunts. Estimate first, then measure.