Finding the volume of a pipe can be useful for any number of reasons, such as calculating the water capacity a pipe can handle. Because a pipe is little more than a long, slim cylinder, you can use geometry to figure out the pipe's volume. If that proves difficult, you can also use water and a graduated container.
Calculate the radius. Measure the diameter of the pipe and divide it by two. The diameter is the distance from one inside edge, across the center and to the opposite inside edge.
Measure the length, or height, of the pipe with a ruler or tape measure. Measure using the same units you used to determine the radius.
Insert the values you found in Steps 1 and 2 for the radius and height into the volume formula for a cylinder: volume = pi x radius squared x height. If the calculator does not have a pi button, use 3.14 as an approximation.
Place a tight-fitting cap on one end of the pipe and fill the pipe with water. This represents an alternate method to find the volume.
Pour the liquid into a graduated container or bowl. The volume of the water equals the volume of the pipe.
TL;DR (Too Long; Didn't Read)
Use the same units for all measurements. If the measurements are in centimeters, the volume will be cubic centimeters. If it is in inches, the volume will be cubic inches.
When measuring the diameter, make sure to measure the inside diameter of the pipe. Otherwise, your volume will come out larger than expected.