NPT is American National Standard Taper Pipe Thread. The thread is cut on a 1 in 16 taper, about 1.79 degrees, so the joint tightens and seals as it is threaded in. Sealant or tape is still normally required.

Why is 1/2 inch NPT actually 0.840 inches across?

Pipe sizes are nominal, not measured dimensions. They originally described the approximate internal diameter of the pipe, so the thread outside diameter is always noticeably larger than the size label.

How many threads per inch does NPT use?

27 TPI for 1/16 and 1/8 inch, 18 TPI for 1/4 and 3/8, 14 TPI for 1/2 and 3/4, 11.5 TPI from 1 inch through 2 inch, and 8 TPI from 2-1/2 inch up.

Is NPT the same as BSP?

No. BSP uses a 55 degree thread form; NPT uses 60 degrees, and the thread pitches differ at most sizes. They are not interchangeable and forcing them together damages both parts.

Which pipe threads apply to electrical conduit?

Rigid metal conduit and IMC use NPT-style tapered threads at the same nominal trade sizes, which is why an RMC die set matches pipe threading equipment.

Pipe Thread Chart

NPT tapered pipe thread dimensions, thread pitch and tap drill sizes for every common nominal size.

Quick answer

NPT threads are cut on a 1 in 16 taper with a 60 degree thread form. Pitches are 27 TPI up to 1/8 in, 18 TPI for 1/4 and 3/8, 14 TPI for 1/2 and 3/4, 11.5 TPI from 1 to 2 in, and 8 TPI above that. Nominal size is a label, not a measurement: 1/2 in NPT measures 0.840 in across the threads.

NPT thread dimensions — ASME B1.20.1

Nominal size	Outside diameter (in)	Threads per inch	Effective thread length (in)	Tap drill
1/16"	0.313	27	0.26	1/4"
1/8"	0.405	27	0.26	11/32"
1/4"	0.540	18	0.40	7/16"
3/8"	0.675	18	0.41	37/64"
1/2"	0.840	14	0.53	23/32"
3/4"	1.050	14	0.55	59/64"
1"	1.315	11.5	0.68	1-5/32"
1-1/4"	1.660	11.5	0.71	1-1/2"
1-1/2"	1.900	11.5	0.72	1-47/64"
2"	2.375	11.5	0.76	2-7/32"
2-1/2"	2.875	8	1.14	2-5/8"
3"	3.500	8	1.20	3-1/4"
3-1/2"	4.000	8	1.25	3-3/4"
4"	4.500	8	1.30	4-1/4"

Published standard

Source & method: Tap drill sizes are the standard commercial recommendation and assume a taper pipe reamer is used before tapping.

Frequently asked

What is NPT thread?: NPT is American National Standard Taper Pipe Thread. The thread is cut on a 1 in 16 taper, about 1.79 degrees, so the joint tightens and seals as it is threaded in. Sealant or tape is still normally required.
Why is 1/2 inch NPT actually 0.840 inches across?: Pipe sizes are nominal, not measured dimensions. They originally described the approximate internal diameter of the pipe, so the thread outside diameter is always noticeably larger than the size label.
How many threads per inch does NPT use?: 27 TPI for 1/16 and 1/8 inch, 18 TPI for 1/4 and 3/8, 14 TPI for 1/2 and 3/4, 11.5 TPI from 1 inch through 2 inch, and 8 TPI from 2-1/2 inch up.
Is NPT the same as BSP?: No. BSP uses a 55 degree thread form; NPT uses 60 degrees, and the thread pitches differ at most sizes. They are not interchangeable and forcing them together damages both parts.
Which pipe threads apply to electrical conduit?: Rigid metal conduit and IMC use NPT-style tapered threads at the same nominal trade sizes, which is why an RMC die set matches pipe threading equipment.

Taps and thread sealant

Matched to the NPT sizes above.

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NPT thread dimensions — ASME B1.20.1

Frequently asked

Taps and thread sealant

Taps and thread sealant

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