Measuring Length and Height

What Does Measuring Mean?

When you measure something, you find out how big it is using numbers. You might want to know how long, how tall, or how wide something is.

• Length is how long something is. We often talk about length when something is lying flat, like a pencil, a table, or a ribbon.
• Height is how tall something is. We talk about height when something stands up, like a tree, a door, or you!

Length and height are really the same idea: a distance from one end to the other. We just use different words depending on whether the thing is lying down or standing up.

Why We Need Units

Long ago, people measured things using their feet or their hands. But there was a problem. A grown-up's foot is much bigger than a child's foot! If you said "the table is 6 feet long," nobody knew whose feet you meant.

So people invented units that are always the same size. A unit is a fixed amount we agree to use. The two units you will use most are:

• the centimetre (we write it as cm) — small, about the width of your fingernail
• the metre (we write it as m) — big, about the height of a door handle from the floor

Because units never change, everyone gets the same answer. That is the whole point!

How to Use a Ruler

A ruler is a tool for measuring length. Here is the most important rule:

Always line up the start of your object with the 0 mark, not the edge of the ruler.

Many rulers have a little gap before the 0. If you start at the edge instead of the 0, your answer will be too big. Start at 0, then read the number where your object ends.

Worked example 1. A crayon starts at 0 cm and its tip ends at 8 cm.

The crayon is 8 cm long. Easy! You just read where it ends, because it began at 0.

Worked example 2. A leaf is placed on the ruler. It starts at 2 cm and ends at 9 cm. How long is it?

This time it did not start at 0, so we subtract:

9 cm − 2 cm = 7 cm

The leaf is 7 cm long. The trick is to find the distance between the start and the end.

Centimetres and Metres Together

Sometimes things are too big to measure in centimetres alone. That is when we use metres. Here is the key fact:

100 centimetres = 1 metre

So 1 metre is the same as 100 centimetres laid end to end. Let's look at a table of what each unit suits:

Unit	Best for measuring	Example
Centimetre (cm)	Small things	A pencil, a book, your hand
Metre (m)	Big things	A room, a fence, your height

Worked example 3. Liam is 1 metre and 25 centimetres tall. How many centimetres is that altogether?

One metre is 100 cm, so:

100 cm + 25 cm = 125 cm

Liam is 125 cm tall.

Comparing Lengths

Once we have measured two things, we can compare them. We ask: which is longer? Which is shorter? Which is taller?

Worked example 4. A red ribbon is 30 cm long. A blue ribbon is 45 cm long. Which is longer, and by how much?

The blue ribbon is longer because 45 is more than 30. To find how much longer:

45 cm − 30 cm = 15 cm

The blue ribbon is 15 cm longer than the red one.

Comparing measurements uses the same big-and-small thinking you may know from comparing sizes: big and small. And when you add or subtract measurements, you are using your addition and subtraction skills!

Picking the Right Unit

A clever measurer chooses the unit that fits the job:

• Measuring a ladybird? Use centimetres (it is tiny).
• Measuring a football pitch? Use metres (it is huge).

Using metres for a ladybird would give a silly answer like "0 metres," and using centimetres for a pitch would give a huge, hard number. Match the unit to the size.

Try It Yourself

Grab a ruler or a tape measure and try these:

1. Measure your stuff. Measure a pencil, a spoon, and a book in centimetres. Remember to start at 0! Which is the longest?
2. How tall are you? Ask someone to measure your height in centimetres. Are you more or less than 1 metre (100 cm)?
3. Find and compare. Find two toys. Measure both. Which is longer, and by how many centimetres?
4. Estimate first. Before you measure something, guess its length. Then measure. How close was your guess?

Measuring is a skill you will use your whole life, from building dens to baking cakes. Keep your ruler handy and keep measuring! 📐