Circumference of a circle

A circle has a radius of 5 centimetres. Find the circumference of the circle. The circumference is the edge, all the way round. Take pi as three point one four. Now we roll that edge out flat. It comes off the circle, and straightens into a line. That line is exactly as long as the edge was. We measure that line in diameters. A diameter runs right across the circle, through the middle. 1 diameter, 2 diameters, 3 diameters. 3 diameters is not quite long enough. There's a little bit left over. So the edge is 3 diameters, and a little bit more. Pi is how many diameters fit. 3, and a bit more. So we can write it down. A circumference is pi times the diameter. So we need the diameter. This circle gives the radius. The diameter is the radius, and the radius again. So it's 2 radius lines. 2 times 5 is 10 centimetres. The question tells us to take pi as three point one four. So the circumference is three point one four times 10, which is thirty one point four centimetres.

Your turn. A round mat has a radius of 21 centimetres. Find the circumference of the mat. Take pi as 22 over 7. The question always tells you which value to take for pi. Try it first. Pause if you like. The edge rolls out into a line, the same way. Same rule. A circumference is pi times the diameter. The question gives the radius, so we double it first. 2 times 21 is 42 centimetres. Then 22 over 7, times 42. 42 divided by 7 is 6, so cross out the 7 and make the 42 a 6. Then 22 times 6 is one hundred and thirty two centimetres. A circumference is the edge, rolled out flat. Rolled out, it is 3 diameters and a bit more. That is what pi counts. So a circumference is pi times the diameter, and a radius is only half a diameter.