One way to measure a circle's circumference in inches is to measure around the circle, but all that bending might break your ruler. A simpler way is to take advantage of circular properties, like the math constant pi. Pi, also known as π, is one of the most important constants. A ratio of a circle's circumference to its diameter, pi never terminates but is often given as just 3.142. You can calculate the circle's circumference in inches with pi in the equations circumference = 2 * radius * pi and circumference = diameter * pi.

Locate the circle's center, and measure the length from its center to a point on its edge to find the radius. For an example, suppose the measurement is 5 inches.

Double the length to calculate the circle's diameter. For this example, multiplying the length of 5 inches by 2 results in 10 inches.

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Multiply the measurement calculated in the previous step by pi. To conclude this example, multiplying 10 by pi results in 31.42. The circle's circumference is 31.42 inches.

#### Tip

Alternatively, you can measure a straight line that travels between two points on the circle's circumference and passes through its center to obtain the diameter and skip to Step 3.