A circle has diameter 12 cm. Work out its circumference.

Step by step

The question

Question: find the circumference of a circle with diameter 12 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 = 12.

π times 12 is written 12π.

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: 12 times 314 is 3768.

3.14 is 314 hundredths.

So the answer is 3768 hundredths.

That is 2 digits after the point: 37.68.

So the circumference is about 37.68 cm.

Let's look back.

First: use the rule.

Last: about how much.