How to Determine the Size of 1/4” and 1/8” National Pipe Thread tapered (NPT) Fittings

Restek’s Technical Service team gets quite a few questions from customers about fittings. There is a bewildering assortment of fitting types and sizes on the Restek web site. A blog post titled, “I need a fitting, but which one?” is where we often direct customers for help with these questions. One of the things discussed in that blog post is National Pipe Thread tapered (NPT) fittings as one of the main types used, but many people have difficulty identifying the correct size NPT fitting for their needs. The confusion comes from the fact that the outside diameter (OD) of an NPT fitting does not match the “name” of the fitting. For an example let’s look at Restek catalog # 23187 shown below, which is a 1/4" to 1/8" NPT Male Connector.

The 1/4" designation in the name refers to the compression fitting side of the fitting (on the left in the picture above) which has a nut and ferrules to accept and connect to 1/4" OD tubing. Makes sense, right? However, the threads on the right side of the fitting in the picture above are called 1/8" Male NPT, but if you measure the OD of the 1/8” NPT side you will find it is about 0.4" in diameter…certainly not very close to 1/8" (0.125”). As a general “rule of thumb” an NPT thread is approximately 1/4" (0.25”) larger than its “name.” For a 1/4" NPT fitting the “nominal” OD is 0.533”.

NPT fittings are slightly tapered so the “nominal” diameter is the diameter in the middle of the threaded portion, as measured by the top (crest) of the threads. This is a bit confusing, but NPT threads are made to the ANSI B1.20.1, SAE AS71051 standard and anything complying with a standard with a name like that is bound to be bewildering. Hopefully the image below will help.

The charts below are from the Swagelok Thread and End Connection Identification Guide with the first chart showing dimensions for male NPT threads and the second chart containing the dimensions for female NPT threads.

 

Finally, you should always use PTFE tape when making a connection with an NPT fitting.

 

Thanks for reading!