ASME B1.20.1 · NPT thread
Threads per inch and approximate tap drill size for National Pipe Taper (NPT) threads, 1/8" through 4" — the reference for threading and tapping pipe.
| Pipe size (NPS) | Threads / inch (TPI) | Approx. tap drill |
|---|---|---|
| 1/16" | 27 | 1/4" (0.250) |
| 1/8" | 27 | 11/32" (0.344) |
| 1/4" | 18 | 7/16" (0.438) |
| 3/8" | 18 | 37/64" (0.578) |
| 1/2" | 14 | 45/64" (0.703) |
| 3/4" | 14 | 59/64" (0.922) |
| 1" | 11.5 | 1-5/32" (1.156) |
| 1-1/4" | 11.5 | 1-1/2" (1.500) |
| 1-1/2" | 11.5 | 1-23/32" (1.719) |
| 2" | 11.5 | 2-3/16" (2.188) |
| 2-1/2" | 8 | 2-9/16" (2.563) |
| 3" | 8 | 3-3/16" (3.188) |
| 4" | 8 | 4-3/16" (4.188) |
NPT (ANSI/ASME B1.20.1) tapered pipe thread. TPI values are standard; tap drill sizes are approximate common values — for tapped holes, confirm with the tap manufacturer's recommendation for your material and required thread engagement.
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NPT (National Pipe Taper, ANSI/ASME B1.20.1) is the standard U.S. pipe thread. Its 1.79° taper means the threads wedge tighter as the joint is made up, sealing — typically with PTFE tape or pipe dope. The threads per inch step down as pipe gets larger: 27 for 1/8", 14 for 1/2"–3/4", 11.5 for 1"–2", and 8 for 2-1/2" and up.
For tapping a female thread, the tap drill sizes above get you close — but they vary with material and the thread engagement you need, so confirm with the tap manufacturer. NPT is not interchangeable with straight (NPS/NPSM) threads even though they look similar.
For pipe dimensions behind the threads, see the pipe schedule chart. Field PM keeps mechanical QA/QC, pressure tests, and weld records together so threaded and welded joints are documented through closeout.
NPT (National Pipe Taper) is the standard U.S. tapered pipe thread (ANSI/ASME B1.20.1). The 1.79° taper wedges the threads together as they tighten, creating a seal — usually with thread sealant or PTFE tape. NPT is not interchangeable with the straight NPS/NPSM thread used on some fittings.
1/2" and 3/4" NPT are 14 threads per inch. 1/8" is 27 TPI, 1/4" and 3/8" are 18, the 1" through 2" sizes are 11.5, and 2-1/2" and larger are 8 TPI.
A common tap drill for 1/2" NPT is 45/64" (0.703"). Tap drill sizes vary slightly by reference and required thread engagement, so verify with the tap manufacturer for your material.
A rule of thumb is hand-tight plus 2 to 3 turns with a wrench for most sizes, but this depends on the fitting, sealant, and pressure. Tighten to a leak-free joint without over-stressing the female fitting.
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