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NPT Thread Chart (TPI and Taper)

Threads per inch for American National Standard Taper Pipe Threads by nominal size, the taper and thread angle, and how to tell NPT from straight pipe threads.

Ref PV-0051ASME B1.20.1

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

NPT (National Pipe Taper) is the standard tapered pipe thread in North America, defined in ASME B1.20.1. The thread is cut on a cone, so it gets tighter as it is screwed in. The taper is 1 in 16 on the diameter, which is 3/4 in per foot. The thread form has a 60° included angle. Tapered threads seal by wedging, with thread sealant or tape filling the small spiral gap at the crests and roots.

How to identify

Measure the outside diameter of the male thread. It is close to the pipe OD for that size, not the nominal size.
Count threads per inch (TPI) with a thread gauge or a ruler.
Check for taper. A taper thread looks slightly cone-shaped and gets tight after a few turns by hand.
Match the OD and TPI to the chart.

Threads per inch by size

Nominal size (in)	TPI	Approx. male thread OD (in)
1/16	27	0.31
1/8	27	0.41
1/4	18	0.54
3/8	18	0.68
1/2	14	0.84
3/4	14	1.05
1	11-1/2	1.32
1-1/4	11-1/2	1.66
1-1/2	11-1/2	1.90
2	11-1/2	2.38
2-1/2	8	2.88
3	8	3.50
4	8	4.50
6	8	6.63

All sizes 2-1/2 and larger use 8 TPI. OD values are rounded pipe ODs; measured thread ODs vary along the taper.

Key points

NPTF (dryseal) has the same TPI and taper but controls crest and root fit to seal without sealant in some uses. It is defined in ASME B1.20.3.
NPS straight pipe threads, such as NPSM, have the same TPI but no taper. They seal with a gasket or O-ring, not the threads.
BSPT and BSPP (British) threads use a 55° angle and different TPI in many sizes. They do not reliably seal with NPT.
The length of thread engagement for each size is listed in ASME B1.20.1. Use those tables for gauging.
Thread sealants must be compatible with the fluid and listed for the service where required.

Common mistakes

Calling a 1/2 in NPT thread "1/2 inch" because it measures 1/2 in. It measures about 0.84 in.
Mating an NPT male with a BSPT female because they "almost fit."
Using NPT fittings with straight-thread ports that need an O-ring.
Over-tightening tapered threads and cracking the female fitting.

Governing standards: ASME B1.20.1 (pipe threads, general purpose); ASME B1.20.3 (dryseal pipe threads).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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