Circumference is the distance, or perimeter, around a circular object. Circumference (C), diameter (d), and radius (r) are related as follows: C = pi × d = pi × 2r, where pi is the ratio of the circumference of a circle to its diameter (that is, pi = C ÷ d). Because this circumference to diameter ratio is constant, pi is constant and approximated by the number 3.14.

Use a measuring device (either a ruler or tape measure) to measure the distance from the edge of one side of the wheel to the edge of the other side, making sure that you cross through the centre of the wheel along the way. This distance from a point on the circle through the centre to a point on the other side of the circle is referred to as the diameter.

Calculate the circumference by putting the following into a calculator: 3.14 × (measurement of diameter).

Hit return on the calculator or the equal key to receive the value of the previous product. Depending on the units you measured your diameter in, the number on your calculator represents the circumference of the wheel in those units. That is, C = pi × d = (3.14) × (measurement of diameter).

Here's a real-world example: Let's say you want to wrap fabric around a bicycle wheel in your house for decoration, but you are not sure what length of fabric you should buy. First, measure the diameter of the wheel. Let's say it's 14.0 inches. Then plug that into your formula C = pi × diameter = 3.14 × (14.0 in.) = 43.96 in. Therefore, you should buy about 44 inches of fabric to cover the circumference of your wheel.

#### Warning

The accuracy of your circumference depends on the accuracy of your measurement. Make sure your diameter measurement crosses through the centre of your wheel.