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NPT Tap Drill Chart
NPT tap drill sizes for every pipe thread from 1/16" to 4", with decimal and metric equivalents, TPI and hand-tight engagement turns.
Drill these before running a taper pipe tap. Because the tap is tapered, the drill size is a single value rather than a percentage-of-thread choice — engagement is set by how far you run the tap in, not by the hole.
| Size | TPI | Tap drill | Decimal | Metric | Hand-tight turns |
|---|---|---|---|---|---|
| 1/16″ | 27 | C | 0.2420″ | 6.15 mm | ~4.3 |
| 1/8″ | 27 | R | 0.3390″ | 8.61 mm | ~4.3 |
| 1/4″ | 18 | 7/16 | 0.4375″ | 11.11 mm | ~4.1 |
| 3/8″ | 18 | 37/64 | 0.5781″ | 14.68 mm | ~4.3 |
| 1/2″ | 14 | 23/32 | 0.7188″ | 18.26 mm | ~4.5 |
| 3/4″ | 14 | 59/64 | 0.9219″ | 23.42 mm | ~4.7 |
| 1″ | 11.5 | 1-5/32 | 1.1562″ | 29.37 mm | ~4.6 |
| 1-1/4″ | 11.5 | 1-1/2 | 1.5000″ | 38.1 mm | ~4.8 |
| 1-1/2″ | 11.5 | 1-47/64 | 1.7344″ | 44.05 mm | ~4.8 |
| 2″ | 11.5 | 2-7/32 | 2.2188″ | 56.36 mm | ~5.0 |
| 2-1/2″ | 8 | 2-5/8 | 2.6250″ | 66.67 mm | ~5.5 |
| 3″ | 8 | 3-1/4 | 3.2500″ | 82.55 mm | ~6.1 |
| 4″ | 8 | 4-1/4 | 4.2500″ | 107.95 mm | ~6.8 |
All of these threads are tapered 3/4″ per foot on diameter (1:16), which is 1°47′ of included taper. Tap drill sizes are
starting points; material and tap maker both shift them slightly.