NPT Pipe Thread (ANSI/ASME B1.20.1)
Direct Answer
NPT (National Pipe Taper) is the standard North American tapered pipe thread governed by ANSI/ASME B1.20.1. Featuring a 60° thread flank angle and a 1:16 taper rate (0.75 inches per foot), NPT achieves sealing through flank wedging and deformation, requiring PTFE tape or pipe sealant to prevent spiral leak paths.
Source: ANSI/ASME B1.20.1-2013
Specs
| Standard Governing Body | ANSI / ASME B1.20.1 |
| Thread Form & Flank Angle | 60° symmetric truncated V-thread |
| Taper Ratio | 1:16 (0.75 in/ft or 1° 47' with centerline) |
| Common Pitches (TPI) | 1/8": 27 TPI | 1/4"-3/8": 18 TPI | 1/2"-3/4": 14 TPI | 1"-2": 11.5 TPI |
| Sealing Mechanism | Thread flank interference fit + PTFE tape / anaerobic sealant |
| Pressure Rating | Up to 5,000–10,000 PSI (dependent on pipe schedule/fitting) |
| Primary Geographical Region | United States, Canada, and Mexico |
60° Taper Geometry and Wedging Action
NPT threads are cut on a 1:16 taper ($0.75\text{ in/ft}$) along the pipe axis. As male and female components are screwed together, the diametrical clearance diminishes until the thread flanks wedge solidly against each other. Because standard machining leaves slight clearance at the crests and roots, a continuous spiral clearance exists that must be filled with PTFE thread seal tape or liquid anaerobic pipe sealant to guarantee a pressure-tight seal.
Standard NPT Thread Pitch and Hand-Tight Engagement
Standard nominal NPT sizes step through distinct thread pitches:
- $1/8\text{-inch NPT}$: $27\text{ TPI}$ (pitch $0.941\text{ mm}$, hand-tight engagement $4.1\text{ mm}$)
- $1/4\text{-inch}$ & $3/8\text{-inch NPT}$: $18\text{ TPI}$ (pitch $1.411\text{ mm}$)
- $1/2\text{-inch}$ & $3/4\text{-inch NPT}$: $14\text{ TPI}$ (pitch $1.814\text{ mm}$, standard domestic water/gas lines)
- $1\text{-inch}$ through $2\text{-inch NPT}$: $11.5\text{ TPI}$ (pitch $2.209\text{ mm}$)
- $2\text{-}1/2\text{-inch}$ and larger: $8\text{ TPI}$ (pitch $3.175\text{ mm}$)
Crucial Note: NPT threads are incompatible with parallel Whitworth (BSPP) threads; cross-threading will gall the threads and produce immediate leaks due to the flank angle mismatch ($60^\circ$ vs. $55^\circ$).