ASME B1.20.1 · ASME B1.20.3
A 60 degree tapered pipe thread that seals on the thread flanks, normally with sealant or PTFE tape. NPTF is the dryseal version with crest and root interference.
10 sizes60 degree flanksTapered thread, 1 in 16Tapered thread, sealed on the thread
No cone, no groove, no flat sealing face - just a thread that visibly narrows towards the end of the fitting.
Where it seals: The thread flanks wedging together, normally with a thread sealant or PTFE tape. The seal is the thread.
A tapered thread seals by wedging, so it strips the port if it is over-tightened chasing a leak. Use sealant or PTFE tape, and stop at the published turns past finger tight rather than at a feel.
Sized by trade size, which is neither the thread diameter nor the bore: 1/4 NPT measures about 0.54 in across the threads.
Where you will find it: Low and medium pressure plumbing, pneumatics, instrumentation, gauge ports and older hydraulic equipment.
Also searched as: NPT, NPTF, pipe thread, MPT, FPT, MIP.
All sizes
| Size | Thread | TPI / pitch | Basic major Ø | Measured across crests | Tube / trade size | Working pressure |
|---|---|---|---|---|---|---|
| 1/16 NPT | 1/16-27 NPT | 27 TPI (0.941 mm pitch) | 7.94 mm / 0.313 in | Not published | 1/16 in trade size | Manufacturer-specific |
| 1/8 NPT | 1/8-27 NPT | 27 TPI (0.941 mm pitch) | 10.29 mm / 0.405 in | 9.9 mm to 10.41 mm | 1/8 in trade size | 5,000 psi (345 bar) |
| 1/4 NPT | 1/4-18 NPT | 18 TPI (1.411 mm pitch) | 13.72 mm / 0.54 in | 13.2 mm to 13.9 mm | 1/4 in trade size | Manufacturer-specific |
| 3/8 NPT | 3/8-18 NPT | 18 TPI (1.411 mm pitch) | 17.15 mm / 0.675 in | 16.6 mm to 17.3 mm | 3/8 in trade size | Manufacturer-specific |
| 1/2 NPT | 1/2-14 NPT | 14 TPI (1.814 mm pitch) | 21.34 mm / 0.84 in | 20.6 mm to 21.6 mm | 1/2 in trade size | 3,000 psi (207 bar) |
| 3/4 NPT | 3/4-14 NPT | 14 TPI (1.814 mm pitch) | 26.67 mm / 1.05 in | 26 mm to 27 mm | 3/4 in trade size | 2,500 psi (172 bar) |
| 1 NPT | 1-11 1/2 NPT | 11.5 TPI (2.209 mm pitch) | 33.4 mm / 1.315 in | 32.5 mm to 33.7 mm | 1 in trade size | 2,000 psi (138 bar) |
| 1 1/4 NPT | 1 1/4-11 1/2 NPT | 11.5 TPI (2.209 mm pitch) | 42.16 mm / 1.66 in | 41.2 mm to 42.5 mm | 1 1/4 in trade size | 1,150 psi (79 bar) |
| 1 1/2 NPT | 1 1/2-11 1/2 NPT | 11.5 TPI (2.209 mm pitch) | 48.26 mm / 1.9 in | 47.3 mm to 48.6 mm | 1 1/2 in trade size | 1,000 psi (69 bar) |
| 2 NPT | 2-11 1/2 NPT | 11.5 TPI (2.209 mm pitch) | 60.33 mm / 2.375 in | 59.3 mm to 60.7 mm | 2 in trade size | 1,000 psi (69 bar) |
7 of the 10 sizes carry a published working pressure. Every one of them prints the exact conditions it applies under on its own page - a figure without its material and safety factor is not a rating.
On paper
Dimensions are as defined by ASME B1.20.1, ASME B1.20.3. FittingKey is an independent reference and is not affiliated with, endorsed by or approved by any standards body or fitting manufacturer.
Every value on this page was checked against 8 independent published sources:
| Column | Sources | What they gave |
|---|---|---|
| Basic major diameter | asme-b1201, amesweb-npt, machiningdoctor-npt | The basic major diameter D is the outside diameter of the pipe, read directly from ASME B1.20.1-1983 Table 1 and matched to four decimal places by two independent reproductions of that table. |
| Measured male thread diameter | gerrard-id, gates-idchart, mcgill-id | Three charts measuring a tapered thread at three different planes. The span is wide on purpose - a 1 in 16 taper means the reading depends on where the caliper sits. |
| Working pressure | tompkins, brennan-pressure | Two catalogues, steel, 4 to 1 against minimum burst per SAE J514. They agree from 1/2 upward and disagree by one whole pipe size at 1/4 and 3/8, where no figure is published. |
Sources disagree: NPTF working pressure at 1/4 and 3/8 nominal pipe size.
Both tables state the same basis - steel, 4 to 1 against minimum burst - and agree exactly from 1/2 upward, so the gap is a one-size offset in one of them rather than a difference of design intent. No working pressure is published at 1/4 or 3/8. Take it from the catalogue of the fitting you are installing.