To calculate the circumference of a circle, you must first have at least one other measurement, such as the radius (the distance between any point along the circumference to the centre) or the diameter (the distance across the circle from one point on the circumference to its opposite point).
How to calculate the circumference of a circle
Let's say you have the radius, and you know the radius of your circle is 10cm. All you need to do is multiply the radius by Pi (3.1416 to 4 decimal places) and then times that total by 2. In this instance that calculation would be: (10cm x 3.1416) = 31.416cm x 2 = 62.832 cm. Therefore, the circumference of a circle with a radius of 10cm is 62.832cm.
What If I Only Have the Diameter of the Circle?
We can still use the above calculation, bearing in mind that the diameter of a circle will always be twice the radius.
This means we do not need to multiply by 2. Instead, as the diameter of a circle with a radius of 10cm will be 20cm, we have the following calculation: 3.1416 x 20cm = 62.832cm. As we see, this corresponds with the first calculation.
In case you've forgotten, Pi is a mathematical constant found by diving the circumference of a circle by its diameter. In this instance: 62.832cm divided by 20 = 3.1416 (remember we only used Pi to 4 decimal places).
Watch this film for more help: