How To Measure Pipe Diameter. The equation for pipe diameter is the square root of 4 times the flow rate divided by pi times velocity. Hold the pipe in 1 hand and take a ruler or tape measure in your other hand.
To measure the threads on a ¼ inch male pipe thread (mpt), measure the very outsides of the thread, then subtract ¼ inch of the outside diameter (od) measurement to account for the thickness of the pipe wall on either side. That is the distance across the middle of the pipe, not including the thickness of the pipe walls. When measuring a female pipe thread (fpt),.
To measure diameter using the 'string method' simply take a piece of string and anchor one end of the string to the pipe using a piece of tape.
I would like the ability to measure pipe diameters without having to select a second measurement point. For example, if the od of your pipe is 1.66” (1 1/2” ips), then using the chart you would determine that the nominal diameter is 1 ¼ with 11 ½ threads per inch. For example, a 3/4 npt pipe thread has an outside diameter of 1.050 inches. Measure the pipe (female thread) measure the inside diameter (id) of the pipe and follow the same steps given above.