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.
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.
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.