How to Determine NPT Pipe Sizes by Measurements

The National Pipe Tapered Thread (NPT) was created in 1864 to set a standard of measurement. An NPT pipe has a tapered male end and a threaded female end to join pipes for mechanical and hydraulic applications. Taking proper measurements of the pipe ends allow you to purchase fittings, nipples and caps that fit the pipe correctly and form a tight seal.

Instructions

1

Turn on a pair of digital calipers. Spread the large jaws apart that measure outside diameter (OD).

2

Place the jaws around the threads of a male pipe and close them until they touch the pipe. Read the digital display of the OD.

3

Measure the OD of a pipe with female (internal threads) on the end of the pipe with digital calipers.

4

Lay a pipe with male threads on a flat surface. Pull out the tip of a measuring tape and lock it in position at the 1-inch mark. Align the tape measure tip with the end of the pipe. Count the threads on the pipe in the 1-inch area. Use a pencil point to touch each thread as you count it for an accurate measurement of threads per inch (TPI).

5

Compare your OD measurements and TPI to a pipe thread size chart and choose the size closest to your measurement to obtain the nominal pipe size.