An outside diameter is the dimension that describes the size of a hollow cylindrical object. Pipes are a common example of such cylindrical objects. The outside diameter of a pipe should be always double-checked before installation to make sure of proper fit. Fortunately, this parameter is easy to compute if you know the inner diameter and the wall thickness of the pipe. Alternatively, calculate the outside diameter using the circumference of a pipe and the mathematical constant "pi."

Divide the inner diameter of a pipe by two to find the inner radius. For example, if the inner diameter is 3 inches then the radius = 3 / 2 = 1.5 inches.

Add up the inner radius and the pipe wall thickness to calculate the outside radius. For instance, if the wall thickness is 1/2 inches then the outside radius is 1.5 + 0.5 = 2 inches.

Multiply the outside radius by two to calculate the outside diameter. In this example, the outside diameter is 2 inches x 2 = 4 inches.

Divide the pipe circumference by the constant pi (3.142) to calculate the outside diameter as an alternate method. For example, if the circumference is 12.5 inches then the outside diameter = 12.5 / 3.142 = 3.978 inches.