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Circumference of a circle

The circumference of a circle is the distance around a circle.

Calculating the circumference of a circle involves a constant called pi with the symbol π. The value of π (pi) is approximately 3.14159265358979323846... but usually rounding to 3.142 should be sufficient. (see a mnemonic for π)

The formula for the circumference of a circle is

C = πd (see a mnemonic for this formula)

or

C = 2πr

where C is the circumference, d is the diameter and r is the radius.

If you are given the diameter then use the formula C = πd

If you are given the radius then use the formula C = 2πr

Example 1:

Find the circumference of the circle with a diameter of 8 inches.

Solution:

Step 1: Write down the formula:	C = πd
Step 2: Plug in the value:	C = 8π

Answer: The circumference of the circle is 8π ≈ 25.163 inches.

Example 2:

Find the circumference of the circle with a radius of 5 inches.

Solution:

Step 1: Write down the formula:	C = 2πr
Step 2: Plug in the value:	C = 10π

Answer: The circumference of the circle is 10 π ≈ 31.42 inches.

The following video shows several examples of how to find the circumference of a circle.

The following video shows a fun way to remember the formulas for the cricumference of a circle and the area of a circle.

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