# How To: Calculate the circumference of a circle

This video shows how to calculate the circumference of a given circle. The video first describes the circumference or perimeter of a circle as the distance around the outside of a circle. To find the circumference we need either the radius or the diameter of the circle. When you know the diameter of the circle, the formula to find the circumference denoted by 'C' is 'pi' times the diameter, where 'd' is the diameter and 'pi' is a constant, the approximate value being 3.14. And when you are given the radius, the formula for circumference is pi times 2 times r where 'r' is the radius. The speaker says both the formulas are the same because 2 times radius is equal to the diameter. He then explains an example where the diameter is given as 10 meters and you are asked to find the exact circumference. You can calculate the circumference as C=pi times d. The value is 10pi meters. He says that when you are asked to find the exact circumference, then you are supposed to leave pi as it is and not substitute pi with its approximate value 3.14 while finding the answer. Then he explains another example asking you to find the exact circumference. Here, the radius of the circle is given as 4 miles. He asks you to calculate the circumference using the formula C=pi times 2 times r. The value then becomes 2 times pi times 4 which is 8pi. The circumference of this circle, he says, is 8pi miles. Another example asks you to calculate the approximate circumference of the given circle. Here, the radius is given as 15.3 mm and you are asked to use pi=3.14 in the solution. So he asks you to use the formula C=pi times 2 times radius which is 2 times 15.3 times 3.14. The value works out to be 96.084 with the approximate circumference of this circle being 96mm. He then finally signs off saying that if you want to learn more about circumference of other different circles, there is video part two available.

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