A circle has diameter 18 cm. Work out its circumference.

Step by step

The question

Question: find the circumference of a circle with diameter 18 cm.

1 Use the rule

Walk once around any circle.

It is a bit more than 3 times the way across.

It is about 3.14 times.

This number is called π.

The circumference is the way around the circle.

So it is π times the diameter.

So we write C = π × d.

Now put the numbers in: d = 18.

π times 18 is written 18π.

We keep π in the answer.

That is the exact answer.

2 About how much

To see about how much, put 3.14 for π.

Forget the points: 18 times 314 is 5652.

3.14 is 314 hundredths.

So the answer is 5652 hundredths.

That is 2 digits after the point: 56.52.

So the circumference is about 56.52 cm.

Let's look back.

First: use the rule.

Last: about how much.