NPT tap drill chart
NPT sizes are nominal pipe sizes, not measurements: a 1/8 NPT hole is about a third of an inch across because 1/8 pipe is 0.405 in outside. The drill below is for tapping straight from the drill without a taper pipe reamer. Hand-tight turns tell you how far a fitting should go in before the wrench; then about 3 more turns.
| Size | TPI | Tap drill | Inch | Pipe OD | Hand tight |
|---|---|---|---|---|---|
| 1/8 NPT | 27 | R | 0.33908.61 mm | 0.405 | 4.4 turns |
| 1/4 NPT | 18 | 7/16" | 0.437511.11 mm | 0.540 | 4.1 turns |
| 3/8 NPT | 18 | 37/64" | 0.578114.69 mm | 0.675 | 4.3 turns |
| 1/2 NPT | 14 | 23/32" | 0.718818.26 mm | 0.840 | 4.5 turns |
| 3/4 NPT | 14 | 59/64" | 0.921923.42 mm | 1.050 | 4.8 turns |
| 1 NPT | 11-1/2 | 1-5/32" | 1.156329.37 mm | 1.315 | 4.6 turns |
| 1-1/4 NPT | 11-1/2 | 1-1/2" | 1.500038.10 mm | 1.660 | 4.8 turns |
| 1-1/2 NPT | 11-1/2 | 1-47/64" | 1.734444.05 mm | 1.900 | 4.8 turns |
| 2 NPT | 11-1/2 | 2-7/32" | 2.218856.36 mm | 2.375 | 5 turns |
1/16 NPT is left out until two independent published charts agree on its drill. Drill sizes for tapping with a taper pipe reamer are not listed yet for the same reason. Sources on How we calculate.