Tap & Drill Size Chart
Look up the recommended tap drill (≈75% thread engagement) for common imperial (UNC/UNF) and metric threads, with decimal-inch and millimeter sizes. Searchable, in your browser.
Imperial threads (UNC / UNF)
| Thread | Tap drill | Decimal (in) | (mm) |
|---|---|---|---|
| #4-40 UNC | #43 | 0.0890 | 2.26 |
| #6-32 UNC | #36 | 0.1065 | 2.71 |
| #8-32 UNC | #29 | 0.1360 | 3.45 |
| #10-24 UNC | #25 | 0.1495 | 3.80 |
| #10-32 UNF | #21 | 0.1590 | 4.04 |
| 1/4-20 UNC | #7 | 0.2010 | 5.11 |
| 1/4-28 UNF | #3 | 0.2130 | 5.41 |
| 5/16-18 UNC | F | 0.2570 | 6.53 |
| 5/16-24 UNF | I | 0.2720 | 6.91 |
| 3/8-16 UNC | 5/16" | 0.3125 | 7.94 |
| 3/8-24 UNF | Q | 0.3320 | 8.43 |
| 1/2-13 UNC | 27/64" | 0.4219 | 10.72 |
| 1/2-20 UNF | 29/64" | 0.4531 | 11.51 |
Metric threads (coarse)
| Thread | Tap drill | Decimal (in) | (mm) |
|---|---|---|---|
| M3 × 0.5 | 2.5 mm | 0.0984 | 2.50 |
| M4 × 0.7 | 3.3 mm | 0.1299 | 3.30 |
| M5 × 0.8 | 4.2 mm | 0.1654 | 4.20 |
| M6 × 1.0 | 5.0 mm | 0.1969 | 5.00 |
| M8 × 1.25 | 6.8 mm | 0.2677 | 6.80 |
| M10 × 1.5 | 8.5 mm | 0.3346 | 8.50 |
| M12 × 1.75 | 10.2 mm | 0.4016 | 10.20 |
| M16 × 2.0 | 14.0 mm | 0.5512 | 14.00 |
| M20 × 2.5 | 17.5 mm | 0.6890 | 17.50 |
No thread matches your search.
About tap drills
The tap drill makes the hole a thread is cut into. These sizes target roughly 75% thread engagement, the common shop standard that balances thread strength against the torque needed to tap. A larger hole (lower engagement) taps more easily; a smaller hole gives fuller threads.
Sizes are nominal references for general-purpose threading in steel and aluminium. Always confirm against your tap manufacturer's chart for critical or hard-material work.
What this does
This chart looks up the recommended tap drill (sized for about 75% thread engagement) for common imperial (UNC/UNF) and metric threads, listing the drill in decimal inches and millimeters.
How to use it
- Search or scroll to your thread size.
- Read the recommended tap drill.
- Use the decimal-inch or millimeter value.
- Size up for easier tapping if needed.
How it works
The listed drills target roughly 75% thread engagement, the common shop standard. A larger hole (lower engagement) taps more easily; a smaller hole gives fuller, stronger threads but needs more torque and risks breaking the tap.
Understanding your result
The listed drill targets about 75 percent thread engagement, the usual shop compromise. Drilling a touch larger lowers engagement so the tap cuts more easily; drilling smaller gives fuller, stronger threads but needs more torque and risks snapping the tap. Pick the side that suits the material.
Example
A 1/4-20 UNC thread uses a #7 drill (0.201 inch). An M6 × 1.0 thread uses a 5.0 mm drill.
Sources & methodology
Last updated .
Frequently asked questions
How is the tap drill size chosen?
The listed drills target roughly 75% thread engagement, the common shop standard. A bigger hole (lower engagement) taps more easily; a smaller hole gives fuller, stronger threads but needs more torque.
What does 75% thread engagement mean?
It is how much of the full thread depth is actually cut. About 75% gives most of the strength of a full thread while keeping tapping torque and breakage risk reasonable.
Is my data stored?
No. Everything runs locally in your browser.
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