This unique measuring device helps determine the "pipe size" of the pipe in question quickly and easily. The pipe caliper is a two in one device which measures both Iron Pipe Size and Copper Tubing Size. INSTRUCTIONS: Open the movable wing and place the pipe caliper around the pipe between both wings. Simply close the wings so the pipe is touching both wings and the top of the caliper. Read the measurement on the caliper IPS = Iron Pipe Size CWT = Copper Tubing Size Pipe Calipers are a must have tool for pipe insulators, plumbers, draftsmen, pipe fitters, engineers, and architects.