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It’s a lesson that cost me $8.50, but now I know the difference between GHT and NPT.

I’ll pass the lesson along to you for free.

If you have 3/4″ pipe fittings, know which thread size they are before you buy a connector.

In my case, I had 3/4″ GHT on one side and 3/4″ NPT on the other. I didn’t know there were different thread pitches; I figured I just needed the diameter and I would be good. I got the right diameter, and they fit well enough that I didn’t know they were wrong, but the way water was squirting out I realized something wasn’t right.

GHT is Garden Hose Thread. It is always 3/4″ diameter. Its main use is for garden hoses, but it is also used for washing machines and other things that are not permanent fixtures of the plumbing system.

NPT is National Pipe Thread. It is of various diameters. It is used for permanent fixtures of the plumbing system.

In my case, I was connecting a washing machine spigot to a recirculating pump. The pump used NPT and the washing machine thread size was GHT. To complicate matters, the other side of the pump went to 1/2″ PEX.

Here is how GHT (left) matches with NPT (right):

As you can see, it takes NPT about 5.5 threads where GHT takes 5 threads. It’s annoyingly close because the fittings will screw together for a few turns, enough so that you don’t know something is wrong until you turn on the water.

To make matters even more complicated, they don’t use just the acronyms GHT and NPT. No, they have to use Mips and MHT and FHT and Fips. The Ms and Fs are for male and female. Anything with an ‘H’ is for Hose, and anything with a ‘P’ is for Pipe thread. Match Hs to Hs and Ps to Ps.