Tap drill chart
Tap drill sizes for cutting internal threads, sized for roughly 75% thread engagement. Pick your thread, drill the listed hole, then tap. For metric threads, the tap drill is simply the bolt diameter minus the thread pitch.
Inch threads (UNC / UNF)
| Thread | TPI | Tap drill | Decimal (in) |
|---|---|---|---|
| #4-40 | 40 | #43 | 0.0890 |
| #6-32 | 32 | #36 | 0.1065 |
| #8-32 | 32 | #29 | 0.1360 |
| #10-24 | 24 | #25 | 0.1495 |
| #10-32 | 32 | #21 | 0.1590 |
| 1/4-20 | 20 | #7 | 0.2010 |
| 1/4-28 | 28 | #3 | 0.2130 |
| 5/16-18 | 18 | F | 0.2570 |
| 5/16-24 | 24 | I | 0.2720 |
| 3/8-16 | 16 | 5/16 | 0.3125 |
| 3/8-24 | 24 | Q | 0.3320 |
| 7/16-14 | 14 | U | 0.3680 |
| 1/2-13 | 13 | 27/64 | 0.4219 |
| 1/2-20 | 20 | 29/64 | 0.4531 |
| 5/8-11 | 11 | 17/32 | 0.5312 |
| 3/4-10 | 10 | 21/32 | 0.6562 |
Metric threads (coarse)
| Thread | Pitch (mm) | Tap drill (mm) |
|---|---|---|
| M3 | 0.5 | 2.5 |
| M4 | 0.7 | 3.3 |
| M5 | 0.8 | 4.2 |
| M6 | 1.0 | 5.0 |
| M8 | 1.25 | 6.8 |
| M10 | 1.5 | 8.5 |
| M12 | 1.75 | 10.2 |
| M14 | 2.0 | 12.0 |
| M16 | 2.0 | 14.0 |
Sizes are for about 75% thread, a good strength-to-effort balance. For tougher materials a slightly larger drill (lower thread percentage) cuts tapping force and breaks fewer taps. Always chamfer the hole and use cutting fluid.
Working out a tap drill the long way?
This chart covers the common sizes. For any thread, pitch, or target thread percentage, the Tap Drill Calculator computes the exact drill diameter.
How to use this chart
Find your thread size in the left column, drill the listed hole, then run the tap. The decimal column lets you substitute the nearest drill you have on hand. For metric threads the rule is drill diameter = bolt diameter minus pitch, so an M6 x 1.0 needs a 5.0 mm hole.
75% thread vs other percentages
These drills give about 75% thread engagement. Dropping from 75% to 60% takes far less tapping torque and snaps fewer taps while giving up only a little holding strength. In hard or gummy materials a lower thread percentage is often the smarter choice.
FAQ
What is tap drill size?
It is the hole diameter you drill before cutting an internal thread. Too big and the threads are weak; too small and the tap binds or snaps.
What drill do I use for a 1/4-20 tap?
A #7 drill (0.201 in) gives about 75% thread for 1/4-20 UNC. A 13/64 in drill is a close substitute.
How do I find a metric tap drill?
Subtract the pitch from the diameter. An M8 x 1.25 needs 8 minus 1.25, rounded to a 6.8 mm drill.
Worked example: A 1/4-20 thread uses a #7 drill (0.2010 inch), 3/8-16 uses 5/16 inch, M6 × 1.0 uses 5.0 mm, and M8 × 1.25 uses 6.8 mm.
Common mistakes: Make sure you read the coarse or fine row that matches the thread. Roll-form (forming) taps need a larger drill than cutting taps, for example about 5.5 mm for M6 instead of 5.0 mm. For a blind hole, drill deeper than the thread length to leave room for the tap.
Related: the Thread Engagement Chart, the Drill Size Chart and the Fractional Drill Chart.