The **perimeter** of a circle is most commonly referred to as the **circumference** of a circle and we will use the word circumference for our explanation below.

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**Diameter of a Circle**

Look at this diagram, here you can see the diameter - the distance from one side of the circle to the other side.

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**Calculating the circumference using diameter.**

We now know what the diameter is and the formula to use so now letâ€™s calculate the circumference.

And there you have your answer, the circumference of a 10cm diameter circle is 31.4cm. If you want to be really exact you could leave your answer as 10∏.

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**Radius**

An alternative method for calculating the circumference is by using the radius of the circle. The radius of a circle is the distance from the center to the edge of the circle. The radius equals half the diameter.

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**Calculating the circumference using the radius.**

If we have the radius of a circle then to calculate the circumference we multiply pi (∏) by the radius (r) of a circle and then multiply by 2.

For the circle above we have a radius (r) of 5cm, so lets calculate the circumference:

And again we have the same answer, the circumference of a 5cm radius circle is 31.4cm. Again, if you want to be really exact you could leave your answer as 10∏.