How To Measure Pipe Thread Size. For example, a 1” mpt pipe thread has an outside diameter of 1.315” npt size thread diameter od of mpt (id of fpt will be slightly less than this.) 1/8” 0.405” 1/4” 0.540” In column 3 will be your nominal pipe thread size.
To find it, measure around the circumference of the pipe with flexible measuring tape. The locknut thread is established on the basis of retaining the greatest possible amount of metal thickness between the bottom of the thread and the inside of the pipe. The dimension in column 3 will be your nominal pipe thread size.
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.
*image not to scale, measurements are appoximates only. The dimension in column 3 will be your nominal pipe thread size. Use a tape rule (or vernier gauge) to measure across the outside of the male thread. Measure circumference using a flexible tape and divide the outside circumference by 3.1415.