NPT thread dimensions
The nominal size is a historical reference to the bore of the pipe, not to anything you can measure on the fitting. That is why a 1/2 inch NPT male thread measures 0.840 inch across.
NPT threads are tapered at 1 in 16, which is 3/4 inch per foot of diameter. The taper is what makes the seal: the flanks jam as the thread goes in. That is also why NPT needs sealant on the flanks and why over-tightening splits female ports.
| Nominal size | Thread OD (in) | TPI | Tap drill | Hand-tight engagement (turns) |
|---|---|---|---|---|
| 1/16 | 0.3125 | 27 | D | 4.0 |
| 1/8 | 0.405 | 27 | R | 4.0 |
| 1/4 | 0.540 | 18 | 7/16 | 4.5 |
| 3/8 | 0.675 | 18 | 37/64 | 4.5 |
| 1/2 | 0.840 | 14 | 23/32 | 5.0 |
| 3/4 | 1.050 | 14 | 59/64 | 5.0 |
| 1 | 1.315 | 11.5 | 1 5/32 | 5.5 |
| 1 1/4 | 1.660 | 11.5 | 1 1/2 | 5.5 |
| 1 1/2 | 1.900 | 11.5 | 1 47/64 | 5.5 |
| 2 | 2.375 | 11.5 | 2 7/32 | 6.0 |
How to measure pipe thread
Measure the outside diameter of a male thread with a caliper, at the largest point since the thread is tapered. On a female port, measure the inside diameter at the opening and add about one thread depth.
Then count the threads per inch with a thread gauge, or lay a rule against the thread and count crests over an inch. Match both numbers against the table. Diameter alone is not enough, because 1/8 and 1/16 NPT share 27 TPI and 1/2 and 3/4 share 14.
NPT versus BSPP and NPS
NPS is the straight, parallel version of the same thread form. It has the same diameter and TPI as NPT but no taper, so it seals on a gasket or O-ring at the face rather than on the thread flanks. An NPS male will start into an NPT female and then stop, which is the usual sign of a mismatch.
BSPP is the bigger trap. A 1/2 inch BSPP thread is also 14 TPI, but the thread angle is 55 degrees against 60 for NPT, and BSPP is parallel. The two will thread together for two or three turns and then bind, and the joint will weep under pressure no matter how much sealant goes on.
If a fitting starts easily and then jams early, stop and check the thread form before forcing it.
| Thread | Form | Angle | Seals on |
|---|---|---|---|
| NPT | Tapered | 60° | Thread flanks, with sealant |
| NPS | Parallel | 60° | Gasket or O-ring at the face |
| BSPT | Tapered | 55° | Thread flanks, with sealant |
| BSPP | Parallel | 55° | Bonded seal or O-ring at the face |
