Tap Drill Basics
How to choose tap drill size for metric and imperial threads. Use our tap drill calculator and chart for correct hole diameter.
What is a tap drill?
The tap drill is the drill size used before tapping internal threads. The hole must be smaller than the bolt's major diameter so the tap can form threads. Standard practice aims for about 75% thread depth for a balance of strength and tap life. Use our tap drill calculator for metric threads and the tap drill chart for quick reference.
Tap drill size comparison (metric)
| Thread | Tap drill (mm) | Thread | Tap drill (mm) |
|---|---|---|---|
| M5 x 0.8 | 4.2 | M8 x 1.25 | 6.8 |
| M6 x 1.0 | 5.0 | M10 x 1.5 | 8.5 |
| M6 x 1.25 | 4.8 | M12 x 1.75 | 10.2 |
Imperial tap drill
For UNC/UNF threads, tap drill sizes are often given in fraction or letter/number drill sizes. Check our tap drill chart and use the drill bit converter if you need to switch between mm, inch, and wire sizes.
FAQ
How do I choose tap drill size?
Use a tap drill that gives about 75% thread depth. Metric: hole ≈ major diameter − pitch. Use our tap drill calculator or chart.
What is the tap drill for M8 x 1.25?
Standard tap drill is 6.8 mm. Confirm with our tap drill calculator for your material and thread type.
Can I use a drill size between chart values?
Stick to recommended sizes when possible. Larger = weaker threads; smaller = risk of tap breakage. Use our calculator when in doubt.