Transcript

The circumference of a circle is the distance around its edge. That is, its perimeter. Before learning how to work out the circumference of a circle, look at these two key terms: diameter and radius. Radius, or r, is the distance from the centre of the circle to the edge. Diameter, or d, is the distance from one edge of the circle to the other, passing through the centre of the circle.

You can see the diameter is always double the radius. If you know either the radius or the diameter, you can always work out the other. And you can also work out the circumference. Here is the basic formula you need to work out the circumference of a circle: Circumference = pi x diameter. This can also be written as C = pi x d, or pi d.

Pi, or this symbol (which is the Greek letter used to represent it), is a constant number that is around the value of 3.142. It's a number that goes on for ever, so you tend to shorten it to the more manageable number of 3.14 or 3.142. In technical terms, pi is the number you get if you divide a circle's circumference by its diameter, and it's the same for every circle.

Let's look at an example of calculating circumference. This circle has a diameter of 5 centimetres. You can put this into the formula, so you get C = pi x 5. Look for the pi key on your calculator, or you can use the shortened version, 3.142. So the circumference, C = 3.142 x 5, which equals 15.71 centimetres.

Here's another example, but this time the radius is labelled. How would you calculate the circumference? Remember that the formula for circumference uses diameter, so you'll need to work this out first. Since the radius is 12 centimetres, the diameter will be double this. 12 x 2 = 24 centimetres, so d equals 24 centimetres. Now using the formula, circumference = 3.142 x 24, which equals 75.408.

Now, try the examples in the next activity.