Metric thread tap drill calculator
Find the tap drill size and the minor and pitch diameters for an ISO metric coarse thread.
How it is calculated
Tap drill ≈ D − P. Internal minor diameter D1 = D − 1.0825P. Pitch diameter d2 = D − 0.6495P.
Worked example
M10 × 1.5: tap drill = 10 − 1.5 = 8.5 mm.
Steps
- Enter the nominal diameter.
- Enter the coarse pitch from ISO 262.
- Use the tap drill size; for fine threads use the same formula with their pitch.
Frequently asked questions
Why is the tap drill D minus P?
It gives about 75 % thread engagement, which suits most steel and aluminum parts.
Does this work for fine threads?
Yes, enter the fine pitch. Check the tap drill chart for very hard materials.
Is this exact to the standard?
It is the common approximation. For tolerance classes use the ISO 965 tables.
References
ISO 262 (metric thread selection) and ISO 724 basic profile formulas.
Estimates for information only. Verify critical values with a qualified engineer or your supplier standards.