Shop reference tools
NPT Tap Drill Chart
Quick reference for common NPT pipe tap drill sizes.
NPT Tap Drill Chart lookup
Start with the table, then use the related pages for tapping, clearance holes, pilot holes, and printable shop sheets.
| NPT size | Suggested drill | Decimal inch | Millimeters |
|---|---|---|---|
| 1/8-27 NPT | R | 0.339 in | 8.61 mm |
| 1/4-18 NPT | 7/16 in | 0.438 in | 11.11 mm |
| 3/8-18 NPT | 37/64 in | 0.578 in | 14.68 mm |
| 1/2-14 NPT | 23/32 in | 0.719 in | 18.26 mm |
| 3/4-14 NPT | 59/64 in | 0.922 in | 23.42 mm |
What drill size do I use for NPT threads?
NPT threads use a separate pipe-tap table, not a straight machine-screw tap drill chart. The table above lists common starting values such as R for 1/8-27 NPT, 7/16 in for 1/4-18 NPT, 37/64 in for 3/8-18 NPT, 23/32 in for 1/2-14 NPT, and 59/64 in for 3/4-14 NPT.
Why is NPT separate from UNC, UNF, and metric taps?
NPT is a tapered pipe thread. Hole preparation, thread depth, sealing method, and tap maker guidance matter more than simply matching a nominal diameter to a straight-thread chart.
Should I ream before tapping NPT?
Some NPT workflows use a taper pipe reamer or manufacturer-specific preparation after drilling. Check the tap maker's recommendation and the fitting requirement before treating a drill-only value as the final production setup.