NPT Tap Drill Sizes
When tapping NPT threads, the drill size determines the minor diameter of the internal thread. Because NPT threads are tapered, the drill creates a straight hole that the tapered tap gradually engages — you’ll get full threads near the entrance and partial threads at depth.
| NPT Size | TPI | Tap Drill (in) | Tap Drill (Letter/Fraction) | Drill Decimal (in) |
|---|---|---|---|---|
| 1/16″ | 27 | 0.242 | C | 0.242 |
| 1/8″ | 27 | 0.339 | R | 0.339 |
| 1/4″ | 18 | 0.4375 | 7/16″ | 0.4375 |
| 3/8″ | 18 | 0.5781 | 37/64″ | 0.5781 |
| 1/2″ | 14 | 0.7188 | 23/32″ | 0.7188 |
| 3/4″ | 14 | 0.9219 | 59/64″ | 0.9219 |
| 1″ | 11.5 | 1.1563 | 1-5/32″ | 1.1563 |
| 1-1/4″ | 11.5 | 1.5000 | 1-1/2″ | 1.5000 |
| 1-1/2″ | 11.5 | 1.7344 | 1-47/64″ | 1.7344 |
| 2″ | 11.5 | 2.1875 | 2-3/16″ | 2.1875 |
Tapping Tips for NPT
- Use a tapered tap — NPT taps are ground with the 1°47′ taper built in
- Drill depth matters — drill deep enough for full thread engagement plus room for the tap to start
- Use cutting fluid — especially in stainless and alloy steels
- Don’t bottom out the tap — NPT taps will crack thin-walled parts if driven too deep
- Check with a plug gauge — hand-tight engagement should be within 1–2 turns of gauge flush
For complete NPT dimensions including OD and thread engagement, see NPT Sizes & Dimensions.
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