Iron Pipe Size (IPS) is an old pipe sizing system that came was started in the early 19th century. It remained in effect until after WWII and was mainly used in the US and UK. When IPS was introduced, pipes were cast in halves and welded together. The sizing of IPS refers to the inside diameter of the pipe and is roughly the same as today’s Nominal Pipe Size (NPS). However, the measurement of NPS is not actually the measured size of the inside diameter of the pipe. A male NPT pipe with an outer diameter of 5/8” will have an NPS of 3/8. Similarly, a female NPT pipe with an inner diameter of 5/8” will have an NPS of 3/8.
Say you are putting a new faucet in your kitchen sink. You measure the male pipe and the outer diameter is 1/2”. When you get to the hardware store you find the 1/2 female pipe and go home to install. When you return home, you find that your female pipe is about a 1/4” bigger than your male pipe! A female NPT pipe sized at 1/2 actually has an inside diameter of 3/4″, making it incompatible with your 1/2″ male pipe, unless you like water gushing everywhere except out your new faucet. Make sure to look at a pipe size chart after you measure your pipe so you know what your NPS really is and don’t have to do the drive of shame back to the store to get a different pipe!