How it connects
Male taper thread screws into the female taper until flanks wedge tight; seal relies on thread interference plus PTFE tape or pipe dope to fill the spiral leak path.
Male and female halves
This is the female (internal) half, called FNPT/FPT/FIP; the male half is MNPT/MPT/MIP.
Usually connects to
- Matching nominal-size/pitch male NPT pipe, nipple, fitting or adapter (MNPT/MPT/MIP)
- Same-size male NPTF dryseal thread where compatible with sealant
Common sizes
- NPT nominal pipe sizes span 1/16–24 in; common plumbing sizes 1/8, 1/4, 3/8, 1/2, 3/4, 1, 1-1/4, 1-1/2 and 2 in, with many lines extending to 4 in.
- Examples: 3/8-18, 1/2-14 (about 0.840 in/21.3 mm matching pipe OD), 3/4-14 (1.050 in/26.67 mm) and 1-1/2-11.5.
- Nominal pipe size is not thread diameter or measured female bore; specify size and TPI together.
Where it is used
Residential plumbing/water systemsPool/spa equipmentCompressed airHydraulics/pneumatics/instrumentationIndustrial fluid/gas process piping
Variations
- NPTF (Dryseal, ASME B1.20.3)
- NPSM/NPS straight pipe threads (ASME B1.20.1, not taper)
- NPSM straight mechanical pipe thread
- NPSC/NPSM straight pipe threads (parallel, not interchangeable for sealing)
How to recognise it
Internal thread in a hex or round boss; bore narrows inward (taper); often marked FNPT/FPT/FIP or with size. No sealing washer or face seal. Thread count matches NPT table (27, 18, 14, 11.5, 8 TPI).
Often confused with
| Confused with | How to tell them apart |
|---|---|
| BSPT / BSPP (R/Rp/G) | BSP uses a 55° thread form vs NPT's 60°, and pitch differs at some sizes (e.g. 1" BSP 11 TPI vs NPT 11.5). Check markings or use a gauge. BSPT has a 55° thread profile versus NPT’s 60°; check pitch and standard, not just apparent fit. |
| NPTF dryseal | Same taper and pitch but controlled crest/root for sealant-free sealing; usually labeled NPTF, common in hydraulics/fuel. |
| GHT garden hose thread | GHT is straight 3/4"-11.5 with a washer seal; NPT is tapered and has no washer. |
| NPS straight pipe thread | NPS has parallel threads; NPT has a 1:16 diameter taper. |
| BSPT / BSPP (ISO 7 / ISO 228) | BSP has 55° thread angle and different pitch in most sizes (1/2 in BSP = 14 TPI too, but 3/4 in BSP is 14 vs NPT 14; 1 in BSP 11 vs NPT 11.5); check angle/marking. |
Also called
FNPT/NPT(F)/Female NPTFPT (female pipe thread)FIP (female iron pipe thread, in US NPT fitting catalogs)IPS female threadFemale National Pipe Taper
Good to know
NPT and NPTF are distinct specifications despite mechanical intermatability. FIP on angle stops and FNPT on pool-pump unions describe the same female NPT thread.
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