12-8 NPT Thread Dimensions and Tap Drill
12 NPT, written in full as 12-8 NPT, is the American taper pipe thread for nominal 12 inch pipe: 8 threads per inch cut on a 1 in 16 taper, on pipe with an outside diameter of 12.750 in (323.85 mm). The nominal size names the pipe and is not a dimension you can measure on the thread.
- Bore before tapping12.4328 in315.79 mm
- Threads per inch8pitch 3.175 mm
- Pipe outside diameter12.750 in323.85 mm
- Pitch Ø at hand-tight plane12.61781 in320.492 mm
- Hand-tight engagement1.360 in10.88 threads
- Effective thread length2.1250 in17.00 threads
Where the diameters are measured
A taper thread has no single pitch diameter. ASME B1.20.1 defines it at the end of the pipe (E0), at the plane where a fitting stops when screwed on by hand (E1), and at the end of the effective thread (E2). Each step of 1 in along the axis adds 1/16 in to the diameter.
12-8 NPT dimensions
| Dimension | in | mm |
|---|---|---|
| Pipe outside diameter D | 12.7500 | 323.850 |
| Pitch P | 0.12500 | 3.175 |
| Pitch diameter at pipe end E0 | 12.53281 | 318.333 |
| Pitch diameter at hand-tight plane E1 | 12.61781 | 320.492 |
| Pitch diameter at end of effective thread E2 | 12.66562 | 321.707 |
| Hand-tight engagement L1 | 1.3600 | 34.544 |
| Effective thread length L2 | 2.1250 | 53.975 |
| Minor diameter at pipe end K0 | 12.4328 | 315.793 |
| Thread height h (0.8 × P) | 0.10000 | 2.540 |
| Property | Value |
|---|---|
| Threads per inch | 8 |
| Flank angle | 60° (30° each side) |
| Taper | 1 in 16 on diameter (3/4 in per foot) |
| Taper angle to the axis | 1° 47′ (1.7899°) |
| Threads engaged by hand | 10.88 |
| Wrench make-up | 2 threads |
Basic dimensions from ASME B1.20.1 Table 2. Millimetre values are conversions.
12 NPT tap drill size
Bore before tapping 12 NPT: 12.4328 in (315.79 mm)
This is K0, the basic minor diameter at the small end of the thread.
No tap drill is published for 12 NPT. Threads this large are bored or milled, and ASME B1.20.1 gives the minor diameter K0 as the basis for the hole size. Taper the bore at 1 in 16 where full threads are needed over the whole engagement.
How far 12 NPT screws in
A 12 NPT fitting turns in by hand for 1.360 in (34.5 mm), about 10.9 threads, before the flanks meet. The standard then allows 2 more threads of wrench make-up, for a total engagement of about 1.610 in (40.9 mm). Tolerance on the hand-tight position is one thread either way, so a joint that stops a turn early or late is still within the standard.
NPT threads do not seal by themselves. The crests and roots leave a spiral gap, so ASME B1.20.1 calls for a sealant (PTFE tape or pipe compound) where the joint must be pressure-tight. Dryseal NPTF threads to ASME B1.20.3 have the same pitch and taper but controlled crests and roots that seal without it.
Related sizes and tools
Sources: ASME B1.20.1 (dimensions, Table 2; suggested drills, Appendix), Machinery’s Handbook and tap makers’ charts (shop tap drill sizes). How this site checks its data. Last reviewed 29 September 2026.