NPT thread: tap drill size chart
Full chart with the tap drill for every size. Press “Calculate” on any row to open the calculator with that thread.
NPT is the American tapered pipe thread (ASME B1.20.1): it seals as the taper is tightened, with PTFE tape or sealant. The nominal size is not any diameter on the part: a 1/4" NPT measures about 13.7 mm across the outside. It is common in pneumatics, hydraulics and equipment of US origin.
| Thread | Threads per inch | Outer Ø | Drill, no reaming | Drill with taper reamer | Calculate |
|---|---|---|---|---|---|
| 1/16-27 NPT | 27 | 0.3125" · 7.938 mm | 1/4" (6.35 mm) | 15/64" (5.95 mm) | |
| 1/8-27 NPT | 27 | 0.405" · 10.287 mm | 11/32" (8.73 mm) | 21/64" (8.33 mm) | |
| 1/4-18 NPT | 18 | 0.54" · 13.716 mm | 7/16" (11.11 mm) | 27/64" (10.72 mm) | |
| 3/8-18 NPT | 18 | 0.675" · 17.145 mm | 37/64" (14.68 mm) | 9/16" (14.29 mm) | |
| 1/2-14 NPT | 14 | 0.84" · 21.336 mm | 23/32" (18.26 mm) | 45/64" (17.86 mm) | |
| 3/4-14 NPT | 14 | 1.05" · 26.67 mm | 59/64" (23.42 mm) | 29/32" (23.02 mm) | |
| 1-11.5 NPT | 11.5 | 1.315" · 33.401 mm | 1-5/32" (29.37 mm) | 1-9/64" (28.97 mm) |
How to read the chart
The drill for a pipe thread does not come from subtracting the pitch: on tapered threads the diameter changes along the hole. That is why the drill from the tap makers' tables is used.
Without reaming: the drill you tap straight after, which is the usual way. With a taper reamer: a slightly smaller drill, because the taper is opened with the reamer before tapping; the tap does less work and the thread comes out cleaner.
"Calculate" opens the calculator with that size. Guide values; on critical parts or parts under pressure, check against the tap maker's data sheet and with a gauge.
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