NPT pipe thread chart.
The NPT size is the pipe size, not the thread diameter. A 1/2 in NPT thread has 14 threads per inch and is cut on pipe with a 0.840 in OD. All NPT threads taper 1 in 16 on the diameter, and the thread angle is 60°.
Threads per inch, pipe outside diameter, pitch diameters and thread lengths for American Standard taper pipe threads, 1/16 in to 4 in.
NPT thread dimensions
Basic dimensions from the American Standard for taper pipe threads (NPT). All values are in inches unless marked.
| Size | TPI | Pitch, in | Pipe OD, in (mm) | E0, in | E1, in | L1 hand-tight, in | L2 effective, in | Turns hand-tight |
|---|---|---|---|---|---|---|---|---|
| 1/16 | 27 | 0.03704 | 0.3125 (7.9) | 0.27118 | 0.28118 | 0.1600 | 0.2611 | 4.3 |
| 1/8 | 27 | 0.03704 | 0.405 (10.3) | 0.36351 | 0.37360 | 0.1615 | 0.2639 | 4.4 |
| 1/4 | 18 | 0.05556 | 0.540 (13.7) | 0.47739 | 0.49163 | 0.2278 | 0.4018 | 4.1 |
| 3/8 | 18 | 0.05556 | 0.675 (17.1) | 0.61201 | 0.62701 | 0.2400 | 0.4078 | 4.3 |
| 1/2 | 14 | 0.07143 | 0.840 (21.3) | 0.75843 | 0.77843 | 0.3200 | 0.5337 | 4.5 |
| 3/4 | 14 | 0.07143 | 1.050 (26.7) | 0.96768 | 0.98887 | 0.3390 | 0.5457 | 4.7 |
| 1 | 11.5 | 0.08696 | 1.315 (33.4) | 1.21363 | 1.23863 | 0.4000 | 0.6828 | 4.6 |
| 1-1/4 | 11.5 | 0.08696 | 1.660 (42.2) | 1.55713 | 1.58338 | 0.4200 | 0.7068 | 4.8 |
| 1-1/2 | 11.5 | 0.08696 | 1.900 (48.3) | 1.79609 | 1.82234 | 0.4200 | 0.7235 | 4.8 |
| 2 | 11.5 | 0.08696 | 2.375 (60.3) | 2.26902 | 2.29627 | 0.4360 | 0.7565 | 5.0 |
| 2-1/2 | 8 | 0.12500 | 2.875 (73.0) | 2.71953 | 2.76216 | 0.6820 | 1.1375 | 5.5 |
| 3 | 8 | 0.12500 | 3.500 (88.9) | 3.34062 | 3.38850 | 0.7660 | 1.2000 | 6.1 |
| 3-1/2 | 8 | 0.12500 | 4.000 (101.6) | 3.83750 | 3.88881 | 0.8210 | 1.2500 | 6.6 |
| 4 | 8 | 0.12500 | 4.500 (114.3) | 4.33438 | 4.38712 | 0.8440 | 1.3000 | 6.8 |
- TPI: threads per inch. Pitch = 1 ÷ TPI.
- Pipe OD: outside diameter of the pipe that carries the male thread.
- E0: pitch diameter at the small end of the male thread.
- E1: pitch diameter at the hand-tight plane. E1 = E0 + L1 ÷ 16.
- L1: length of engagement when you tighten the joint by hand.
- L2: effective thread length on the male thread.
- Turns hand-tight = L1 × TPI (calculated).
The NPT formulas
The standard gives formulas for the basic dimensions. The table values come from them:
E0 = D − (0.05 D + 1.1) × p L2 = (0.8 D + 6.8) × p taper = 1 in 16 on the diameter (0.0625 in per in, 3/4 in per ft) thread angle = 60° D = outside diameter of the pipe, p = pitch = 1 ÷ TPI
Example for 1/2 in NPT: D = 0.840 in, p = 1/14 in. E0 = 0.840 − (0.042 + 1.1) ÷ 14 = 0.75843 in. L2 = (0.672 + 6.8) ÷ 14 = 0.5337 in.
How to identify an NPT thread
- Measure the outside diameter of the male thread near the end.
- Find the nearest pipe OD in the chart. The thread is slightly smaller than the pipe OD because of the taper.
- Count the threads in 1 in, or use a thread gauge. Compare with the TPI column.
- Check the taper. An NPT thread gets wider along its length. A straight thread does not.
NPT is not the same as the pipe size. NPS (nominal pipe size) is the size of the pipe. NPT is the thread on it. A 1/2 in NPT thread is cut on 1/2 in NPS pipe, which has a 0.840 in OD. See the pipe size chart.
Standard NPT threads need a thread sealant or tape to make a pressure-tight joint. This chart gives thread dimensions only. It does not give tap drill sizes, because the thread standard does not define them. Use the tap maker's data.
Common questions
What does NPT stand for?
NPT means American National Standard Taper Pipe Thread. It is the standard tapered thread for pipes and fittings in the US.
What size is a 1/2 inch NPT thread?
It is cut on pipe with a 0.840 in (21.3 mm) outside diameter and has 14 threads per inch. The pitch diameter at the small end is 0.758 in.
How many threads per inch is 3/4 NPT?
14 threads per inch, the same as 1/2 in NPT. Sizes 1 in to 2 in have 11.5 TPI, and 2-1/2 in and larger have 8 TPI.
What is the taper of an NPT thread?
The diameter changes by 1 in per 16 in of length. That is 0.0625 in per inch, or 3/4 in per foot. The angle on each side is about 1° 47′.
How many turns is hand-tight for NPT?
The standard hand-tight engagement L1 is 0.320 in for 1/2 in NPT, about 4.5 turns. For 3/4 in it is 0.339 in, about 4.7 turns. Wrench makeup adds more turns.
Are NPT and NPS the same?
No. NPS (nominal pipe size) is the size of the pipe. NPT is the tapered thread cut on that pipe. Straight pipe threads also exist, for example NPSM, but they do not seal on the thread.
Sources
- Basic dimensions (TPI, OD, E0, L1, L2): NBS Handbook H28 (1957), Part II, Table VII.2 — Basic dimensions of American Standard taper pipe threads, NPT.
- Standard: ASME B1.20.1 — Pipe Threads, General Purpose, Inch. The E0 and L2 values above are checked against its formulas.