What Are NPS and NPT Fittings
NPS, NPT, NP...What?
In this article, we're going to explain the differences and similarities between NPS and NPT fittings. First of all, what do they stand for? Both are acronyms for a standard of thread used in the plumbing industry. NPS stands for National Pipe Straight, and NPT stands for National Pipe Tapered.
So what's the difference?
Fundamentally, the difference is in the application. Straight-threaded fittings have walls that are perfectly parallel; the diameter of the threads does not vary from one end of the threaded length to the other end. This allows a fastener or nut to thread up and down freely along the entire length of that thread. Picture a nut on a bolt. You can run it up and down the entire length with no restriction.
A tapered threaded fitting on the other hand has threads that get gradually larger from the open end of the thread to the end of the threaded system. This allows for a secure connection when threaded fittings are mated together. The tapers of the fitting, the female side and the male side match each other, and it actually creates a seal as the fittings thread together that makes for a water, or even air-tight, connection so there is no need to use any adhesives or plumbers tape (picture fittings used in a pneumatic air system).
Measuring NPT and NPS Fittings
It's important to understand that when measuring fittings, you don't actually just use a tape measure to capture the dimensions of the part. Doing this will lead to you purchasing the wrong size fitting. Because these are fundamentally plumbing fittings, the sizes provided are based on the inside diameter of the pipe that a fitting will thread to. For example, a 1" NPT threaded connection will actually measure 1-1/4" with a tape measure.
NPT and NPS Threaded Connection Conversion:
| NPT/NPS Size | Actual Size |
|---|---|
| 1/2" | 3/4" |
| 3/4" | 1" |
| 1" | 1-1/4" |
| 1-1/4" | 1-1/2" |
| 1-1/2" | 1-3/4" |
Which Thread Pattern to Use?
That all depends on what brand ballast bag you have. If you're piecing together your own solution it's something to be aware as NPS fittings will not mate to NPT fittings correctly so the thread pattern, in addition to the thread size, needs to be considered. Check out our Ballast Bag Fittings Article
If you have any questions about threaded connections, and which parts are correct for your application, please contact a product expert.
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