Drill and tap size chart: common UNC, UNF and metric sizes
A tap drill size chart for common UNC, UNF and metric threads (about 75% thread), how to read it, and when to use a different thread percentage.
The tap drill is the hole you drill before cutting an internal thread. For a standard 75% thread, use the tap drill listed for your size: a 1/4-20 UNC tap wants a #7 drill (0.201 in), and an M6 x 1.0 tap wants a 5.0 mm drill. Too small a hole makes tapping hard and snaps taps; too large and the thread is weak. The charts below cover the common inch and metric sizes at roughly 75% thread.
How to read a tap drill chart
The listed tap drill gives about 75% thread engagement, the normal default. That captures most of the thread strength with far less tapping torque (and far fewer broken taps) than a 100% thread. In hard or gummy materials, many shops open the hole up to a 60 to 65% thread to ease tapping; going the other way, to a higher percentage, is rarely worth the risk.
Common inch tap drill sizes (UNC and UNF, ~75% thread)
| Thread | Tap drill | Decimal (in) |
|---|---|---|
| #4-40 | #43 | 0.089 |
| #6-32 | #36 | 0.1065 |
| #8-32 | #29 | 0.136 |
| #10-24 | #25 | 0.1495 |
| #10-32 | #21 | 0.159 |
| 1/4-20 | #7 | 0.201 |
| 1/4-28 | #3 | 0.213 |
| 5/16-18 | F | 0.257 |
| 3/8-16 | 5/16 | 0.3125 |
| 1/2-13 | 27/64 | 0.4219 |
Common metric tap drill sizes (coarse, ~75% thread)
| Thread | Tap drill (mm) |
|---|---|
| M3 x 0.5 | 2.5 |
| M4 x 0.7 | 3.3 |
| M5 x 0.8 | 4.2 |
| M6 x 1.0 | 5.0 |
| M8 x 1.25 | 6.8 |
| M10 x 1.5 | 8.5 |
| M12 x 1.75 | 10.2 |
Thread percentage and material
A quick rule: tap drill size moves with how much thread you want. 75% is the default; drop toward 60 to 65% (a slightly bigger drill) in stainless, titanium and other tough or work-hardening materials to cut tapping torque. The gain in strength from a 100% thread is tiny and not worth the broken taps.
Clearance holes are different
A clearance drill is for a bolt to pass straight through, so it is larger than the bolt, not the tap drill. Do not confuse the two: the tap drill cuts threads, the clearance drill does not.
When to send it to a shop
For tapped holes to a real spec (thread class, depth, and true position of the hole pattern), a machine shop taps to the drawing and inspects it. Request quotes and send the drawing with the thread callouts.
FAQ
What drill size do I use for a 1/4-20 tap?
A #7 drill (0.201 in) for a standard 75% thread. In tough materials you can open it slightly toward a lower thread percentage.
What drill size for an M6 tap?
A 5.0 mm drill for M6 x 1.0 (coarse) at about 75% thread.
What is a tap drill size?
The diameter of the hole drilled before tapping, chosen so the tap cuts the thread to the target engagement, usually 75%.
Why is 75% thread the standard?
It gives almost the full strength of a 100% thread with far less tapping torque and far fewer broken taps, so it is the practical default.
What is the difference between a tap drill and a clearance drill?
The tap drill is sized to cut internal threads; the clearance drill is larger and sized for a bolt to pass through freely.
Last reviewed: October 2026. Related reading: GD&T explained and CNC tolerances and surface finish.
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