A flange is a circular piece of equipment used to connect two pipes together. Flanges can be made out of stainless steel, copper or any other type of metal. They typically match the metal of the pipes they connect. Flanges help seal the pipes together, preventing water from leaking when it passes from one pipe to another along the way from its origin to its destination.
Place one end of a tape measure on the inside of the flange. The diameter is used to determine what size pipe a flange can be used with, so you must find this information by using the inside of the flange as opposed to the outside.
Stretch your tape measure across the center of the circular shape of the flange until it touches the other side of the unit. Keep this end of the tape measure on the inside of the flange as well. Because the flange is circular in shape, the tape measure must stretch across the exact center of the flange to provide an accurate measurement.
Read your tape measure, in inches. The distance you have found is the diameter of your flange.