Pipe Sizes (NPS) & NPT Threads
Nominal pipe size is not the pipe's measured diameter — a “1 inch” pipe is 1.315″ OD. Every NPS has one fixed OD; the schedule sets the wall thickness (and therefore the bore). Enter a measured OD below to identify a pipe, or browse the chart.
Identify a pipe from a measured OD
Decimal inches by default ( ); add “mm” for millimeters (). Wrap a tape around the pipe? Divide the circumference by π first.
NPS dimensions (ASME B36.10M)
| NPS | OD | Sch 40 wall | Sch 40 ID | Sch 80 wall | Sch 80 ID |
|---|---|---|---|---|---|
| 1/8 | 0.405″ / 10.287 mm | 0.068″ | 0.269″ | 0.095″ | 0.215″ |
| 1/4 | 0.540″ / 13.716 mm | 0.088″ | 0.364″ | 0.119″ | 0.302″ |
| 3/8 | 0.675″ / 17.145 mm | 0.091″ | 0.493″ | 0.126″ | 0.423″ |
| 1/2 | 0.840″ / 21.336 mm | 0.109″ | 0.622″ | 0.147″ | 0.546″ |
| 3/4 | 1.050″ / 26.67 mm | 0.113″ | 0.824″ | 0.154″ | 0.742″ |
| 1 | 1.315″ / 33.401 mm | 0.133″ | 1.049″ | 0.179″ | 0.957″ |
| 1-1/4 | 1.660″ / 42.164 mm | 0.140″ | 1.380″ | 0.191″ | 1.278″ |
| 1-1/2 | 1.900″ / 48.26 mm | 0.145″ | 1.610″ | 0.200″ | 1.500″ |
| 2 | 2.375″ / 60.325 mm | 0.154″ | 2.067″ | 0.218″ | 1.939″ |
| 2-1/2 | 2.875″ / 73.025 mm | 0.203″ | 2.469″ | 0.276″ | 2.323″ |
| 3 | 3.500″ / 88.9 mm | 0.216″ | 3.068″ | 0.300″ | 2.900″ |
| 3-1/2 | 4.000″ / 101.6 mm | 0.226″ | 3.548″ | 0.318″ | 3.364″ |
| 4 | 4.500″ / 114.3 mm | 0.237″ | 4.026″ | 0.337″ | 3.826″ |
| 5 | 5.563″ / 141.3 mm | 0.258″ | 5.047″ | 0.375″ | 4.813″ |
| 6 | 6.625″ / 168.275 mm | 0.280″ | 6.065″ | 0.432″ | 5.761″ |
| 8 | 8.625″ / 219.075 mm | 0.322″ | 7.981″ | 0.500″ | 7.625″ |
| 10 | 10.750″ / 273.05 mm | 0.365″ | 10.020″ | 0.594″ | 9.562″ |
| 12 | 12.750″ / 323.85 mm | 0.406″ | 11.938″ | 0.688″ | 11.374″ |
Sch 40 = STD and Sch 80 = XS through NPS 8; they diverge above that (see each size's page — NPS 12 STD is 0.375″ while Sch 40 is 0.406″, a classic specification trap).
NPT taper threads & tap drills
| Thread | TPI | Tap drill | Tap drill Ø |
|---|---|---|---|
| 1/8″ NPT | 27 | R | 0.3390″ |
| 1/4″ NPT | 18 | 7/16″ | 0.4375″ |
| 3/8″ NPT | 18 | 37/64″ | 0.5781″ |
| 1/2″ NPT | 14 | 23/32″ | 0.7188″ |
| 3/4″ NPT | 14 | 59/64″ | 0.9219″ |
| 1″ NPT | 11.5 | 1-5/32″ | 1.1563″ |
| 1-1/4″ NPT | 11.5 | 1-1/2″ | 1.5000″ |
| 1-1/2″ NPT | 11.5 | 1-47/64″ | 1.7344″ |
| 2″ NPT | 11.5 | 2-7/32″ | 2.2188″ |
| 2-1/2″ NPT | 8 | 2-5/8″ | 2.6250″ |
| 3″ NPT | 8 | 3-1/4″ | 3.2500″ |
The standard published chart, assuming usual taper-tap practice. Charts that assume an unreamed hole may list one drill size smaller. NPT threads are tapered — they seal on the threads (with sealant), unlike straight NPS threads.