If you want to replace that leaky pipe by yourself, but are daunted by not knowing what size it is, you can use simple geometry to find its radius. The circular shape of a pipe follows the same geometric rules as two-dimensional circles. A radius is the distance between the center of a circle -- or pipe, in this case -- to its circumference, which is its border. The radius is also half of the diameter, which is the distance between two points on the circumference that passes straight through the circle's center. You can calculate the radius of a pipe through its diameter or circumference.
Halve the measurement of the diameter to calculate the pipe's radius. For example, if the diameter is 20, then halving that length produces a radius of 10.
Divide the circumference of the pipe by π, which is a mathematical constant that begins with 3.14. For example, assume the circumference of the pipe is 60. Dividing 60 by π equals 19.099.
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Divide that number by 2 to calculate the radius. For this example, 19.099 divided by 2 equals 9.5495.