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NPT Tap Drill Size Chart
National Pipe Taper (NPT) threads for plumbing and pneumatic fittings. Because NPT is a taper thread, the tap drill is sized to the pipe's minor diameter at the gauge plane – not to a percent engagement like a machine screw.
NPT tap drill chart
| Thread | TPI | Tap drill | inches | mm |
|---|---|---|---|---|
| 1/8-27 NPT | 27 | 21/64 | 0.3281 | 8.33 |
| 1/4-18 NPT | 18 | 7/16 | 0.4375 | 11.11 |
| 1/2-14 NPT | 14 | 23/32 | 0.7188 | 18.26 |
| 3/8-18 NPT | 18 | 37/64 | 0.5781 | 14.68 |
| 1-1/2-11½ NPT | 11.5 | 1-47/64 | 1.7344 | 44.05 |
About NPT tap drills
Because NPT is a taper thread, each size is drilled to the pipe's internal diameter at the gauge plane rather than to a percentage engagement. The chart above gives the correct drill for the three most common NPT sizes; the linked pages cover the smaller pipe threads too.
For other pipe thread sizes, see the full reference in the related chart pages or use the calculator above.