What is the outer diameter of a pipe thread? Pipes are odd in that their size is based roughly on the inner diameter of the pipe. So when you have a National Pipe Thead – NPT fitting, the thread size is actually much larger than the name implies.
Pipe thread sizes
| Nominal NPT Size |
Outer Diameter inch (mm) |
Threads per inch |
Pitch of Thread inch (mm) |
|---|---|---|---|
| 1/16“ | 0.3125 in (7.94 mm) | 27 | 0.03704 in (0.94082 mm) |
| ⅛” | 0.405 in (10.29 mm) | 27 | 0.03704 in (0.94082 mm) |
| ¼” | 0.540 in (13.72 mm) | 18 | 0.05556 in (1.41122 mm) |
| 3/8″ | 0.675 in (17.15 mm) | 18 | 0.05556 in (1.41122 mm) |
| ½” | 0.840 in (21.34 mm) | 14 | 0.07143 in (1.81432 mm) |
| ¾” | 1.050 in (26.67 mm) | 14 | 0.07143 in (1.81432 mm) |
| 1″ | 1.315 in (33.40 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 1¼” | 1.660 in (42.16 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 1½” | 1.900 in (48.26 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 2″ | 2.375 in (60.33 mm) | 11½ | 0.08696 in (2.20878 mm) |
| 2½” | 2.875 in (73.02 mm) | 8 | 0.12500 in (3.175 mm) |
| 3″ | 3.500 in (88.90 mm) | 8 | 0.12500 in (3.175 mm) |
| 4″ | 4.500 in (114.30 mm) | 8 | 0.12500 in (3.175 mm) |
| 5″ | 5.563 in (141.30 mm) | 8 | 0.12500 in (3.175 mm) |
| 6″ | 6.625 in (168.27 mm) | 8 | 0.12500 in (3.175 mm) |
| 10″ | 10.750 in (273.05 mm) | 8 | 0.12500 in (3.175 mm) |
| 12″ | 12.750 in (323.85 mm) | 8 | 0.12500 in (3.175 mm) |
| 14″ OD | 14 in (355.60 mm) | 8 | 0.12500 in (3.175 mm) |
| 16″ OD | 16 in (406.40 mm) | 8 | 0.12500 in (3.175 mm) |
| 18″ OD | 18 in (457.20 mm) | 8 | 0.12500 in (3.175 mm) |
| 20″ OD | 20 in (508.00 mm) | 8 | 0.12500 in (3.175 mm) |
| 24″ OD | 24 in (609.60 mm) | 8 | 0.12500 in (3.175 mm) |
| Definition of pipe thread acronyms | |
| NPT | National Pipe Thread |
| FPT | Female Pipe Thread |
| MPT | Male Pipe Thread |
| PTF | SAE short taper pipe thread – dryseal |
| NPTF | American National taper pipe thread for dryseal pressure-tight joints |
| NPSM | American National straight pipe thread for mechanical joints |
| NPSI | American National straight intermediate pipe thread |
| GHT | Garden Hose Thread |
| NST | Fire Hose Thread |
| BSPT | British Standard taper pipe thread (metric dimensions) |