What is NPT?
NPT (National Pipe Thread) is the US standard for tapered pipe threads. The taper is 1°47′ (3/4 inch per foot), which means the threads wedge tighter as you screw them together — creating a seal. NPT threads require thread sealant (Teflon tape or pipe dope) for a leak-free connection.
NPT vs. NPTF: NPTF (National Pipe Thread Fuel) is a “dryseal” version that doesn’t require sealant for low-pressure applications. Same dimensions, tighter tolerances on thread form.
NPT Thread Dimensions
| Nominal Pipe Size | Threads Per Inch | OD of Pipe (in) | Tap Drill Size (in) | Thread Engagement (in) |
|---|---|---|---|---|
| 1/16″ | 27 | 0.3125 | C (0.242) | 0.160 |
| 1/8″ | 27 | 0.405 | R (0.339) | 0.160 |
| 1/4″ | 18 | 0.540 | 7/16 (0.4375) | 0.228 |
| 3/8″ | 18 | 0.675 | 37/64 (0.5781) | 0.240 |
| 1/2″ | 14 | 0.840 | 23/32 (0.7188) | 0.320 |
| 3/4″ | 14 | 1.050 | 59/64 (0.9219) | 0.339 |
| 1″ | 11.5 | 1.315 | 1-5/32 (1.1563) | 0.400 |
| 1-1/4″ | 11.5 | 1.660 | 1-1/2 (1.5000) | 0.420 |
| 1-1/2″ | 11.5 | 1.900 | 1-47/64 (1.7344) | 0.420 |
| 2″ | 11.5 | 2.375 | 2-3/16 (2.1875) | 0.436 |
Important Notes
- “Nominal pipe size” is NOT the actual diameter. A 1/2″ NPT fitting has a 0.840″ OD — the name is historical and based on the pipe’s approximate bore, not its external dimension.
- Hand-tight + 1.5–3 turns is the typical assembly spec for NPT connections with Teflon tape.
- Don’t over-tighten — the taper means additional turns create very high hoop stress. Cracking cast fittings is a common mistake.
- NPT is not compatible with BSP (British Standard Pipe) — they look similar but have different thread angles (60° vs 55°).
For tap drill sizes, see NPT Tap Drill Size Chart.