Tap and Drill Size Lookup
The correct tap drill size for common US and metric thread sizes.
| Thread size | Tap drill | Notes |
|---|---|---|
| 1/4-20 | #7 (0.201 in) | 25/64 in reamed |
| 5/16-18 | F (0.257 in) | -- |
| 3/8-16 | 5/16 in (0.3125 in) | -- |
| 7/16-14 | U (0.368 in) | -- |
| 1/2-13 | 27/64 in (0.4219 in) | -- |
| 9/16-12 | 31/64 in (0.4844 in) | -- |
| 5/8-11 | 17/32 in (0.5313 in) | -- |
| M6x1.0 | 5.0 mm | -- |
| M8x1.25 | 6.8 mm | -- |
| M10x1.5 | 8.5 mm | -- |
| M12x1.75 | 10.2 mm | -- |
How to read this chart
The tap drill size leaves the right amount of material for the tap to cut clean, full threads -- roughly 75% thread engagement, which is the generally accepted balance between thread strength and how much torque it takes to cut the thread without breaking the tap. A drill that's too small makes the tap harder to turn and more likely to snap; too large and the resulting threads are weak.
Where these numbers come from
Standard published tap drill charts for UNC (coarse) US threads and standard metric threads, targeting approximately 75% thread engagement. Fine threads (UNF) and specialty threads use different drill sizes than shown here -- always confirm against a full tap drill chart or your tap manufacturer's specification for anything outside these common coarse sizes.
Frequently asked questions
Why not just drill the exact thread diameter?
That would leave no material for the tap to actually cut threads into -- the tap drill is deliberately undersized so the tap can cut full, clean threads in the remaining material.
What happens if I use the wrong drill size?
Too small and the tap is hard to turn and prone to snapping off in the hole; too large and the resulting threads are weak and may strip under load.
Does this chart cover fine threads (UNF)?
No -- this covers common coarse (UNC) threads. Fine threads use different tap drill sizes; check a dedicated UNF chart or your tap's packaging for those.
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