About the Tap Drill Size Calculator
Correct tap drill diameter for a thread, at any target percentage of thread engagement.
Also known as: tapping drill chart, drill size for tap, tap hole size, thread drill size.
How to use this calculator
- Enter nominal thread diameter, thread pitch, target thread engagement.
- The result updates instantly as you type — press Calculate (or Enter) at any time.
- Review the breakdown shown under the main result.
- Use Copy result or Share to save the answer or send a link with your inputs.
Formula
tap drill = D − (engagement % × 1.0825 × pitch)
Worked example
Using the values pre-filled in the calculator above:
- Nominal thread diameter = 10 mm
- Thread pitch = 1.5 mm
- Target thread engagement = 75 %
the calculator returns:
- Tap drill diameter: 8.782 mm
- In inches: 0.3458
- Rule of thumb (D − pitch): 8.5 mm
- Clearance hole (close fit): 10.5 mm
Tips & common mistakes
- 75% engagement gives nearly full strength with far less tapping torque than 100%.