NPT Thread Size Chart (ASME B1.20.1)
Look up NPT (National Pipe Taper) thread sizes used on transmitters, gauges, manifolds and fittings: threads per inch, outside diameter and tap drill.
NPT sizes
| Size | TPI | OD (in) | OD (mm) | Tap drill |
|---|---|---|---|---|
| 1/16" | 27 | 0.313 | 7.94 | 1/4" |
| 1/8" | 27 | 0.405 | 10.29 | 11/32" |
| 1/4" | 18 | 0.540 | 13.72 | 7/16" |
| 3/8" | 18 | 0.675 | 17.15 | 37/64" |
| 1/2" | 14 | 0.840 | 21.34 | 23/32" |
| 3/4" | 14 | 1.050 | 26.67 | 59/64" |
| 1" | 11.5 | 1.315 | 33.40 | 1 5/32" |
| 1 1/4" | 11.5 | 1.660 | 42.16 | 1 1/2" |
| 1 1/2" | 11.5 | 1.900 | 48.26 | 1 47/64" |
| 2" | 11.5 | 2.375 | 60.32 | 2 7/32" |
NPT Thread Basics
The nominal size refers to the pipe, not the thread diameter: a 1/2" NPT thread has an outside diameter of about 0.84 in (21.3 mm).
Worked example
1/2" NPT: 14 TPI, OD 0.840 in (21.34 mm), tap drill 23/32". This is the most common process connection on pressure transmitters and gauges.
Frequently Asked Questions
How many threads per inch is 1/2" NPT?
14 threads per inch.
What is the OD of 1/2" NPT?
0.840 in (21.34 mm), the same as 1/2" pipe OD.
Are NPT and BSPT interchangeable?
No. They have different thread angles (60° and 55°), and 1/2" and 3/4" share 14 TPI but still do not seal properly together.
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