Threaded pipe in North America uses NPT (National Pipe Taper). The threads are tapered to seal, and the nominal size is not the actual outside diameter. This chart gives the key figures.
| Nominal (NPT) | Threads/in | Actual OD |
|---|---|---|
| 1/8 in | 27 | 0.405 |
| 1/4 in | 18 | 0.540 |
| 3/8 in | 18 | 0.675 |
| 1/2 in | 14 | 0.840 |
| 3/4 in | 14 | 1.050 |
| 1 in | 11.5 | 1.315 |
Tapered to seal. NPT threads tighten and wedge as they go, so they need thread sealant tape or compound to be leak-free. The taper is why you cannot fully hand-tighten them.
See the Pipe Schedule Chart and PVC Pipe Dimensions.