Tap Drill Size Calculator
Find correct tap drill size for metric (M2-M30) and imperial (to 1-8 UNC/UNF) threads.
Returns drill diameter in mm and inches at 75% thread engagement.
Selecting the correct tap drill size is essential for creating strong, clean internal threads. The drill must be smaller than the tap to leave material for the threads, but not so small that the tap binds or breaks.
Metric Thread Formula
Tap Drill Diameter = Major Diameter − Pitch
This gives approximately 75% thread engagement, which is the standard recommendation.
For a specific thread percentage:
Tap Drill = Major Diameter − (Pitch × Thread% / 76.98)
Where Thread% is the desired percentage of full thread engagement (typically 60–80%).
Imperial (Unified) Thread Formula
Tap Drill = Major Diameter − (1 / TPI)
Where TPI = threads per inch.
Thread Engagement Reference
| Thread % | Strength | Tap Breakage Risk | Recommended For |
|---|---|---|---|
| 50% | 90% of full | Very low | Aluminum, soft metals |
| 60% | 94% of full | Low | General purpose |
| 75% | 98% of full | Moderate | Standard (default) |
| 83% | 99% of full | High | Steel, critical applications |
| 100% | 100% | Very high | Not recommended |
Key Insight: Going from 75% to 100% thread engagement only gains 2% more strength but dramatically increases tap breakage risk and required torque.
Worked Example — M10×1.5
Major diameter: 10 mm. Pitch: 1.5 mm. Target: 75% thread.
- Tap drill = 10 − (1.5 × 75 / 76.98) = 10 − 1.462 = 8.54 mm
- Nearest standard drill: 8.5 mm
Common Metric Tap Drill Chart
| Thread | Pitch | 75% Drill | Nearest Std |
|---|---|---|---|
| M3 | 0.5 | 2.51 mm | 2.5 mm |
| M4 | 0.7 | 3.32 mm | 3.3 mm |
| M5 | 0.8 | 4.22 mm | 4.2 mm |
| M6 | 1.0 | 5.03 mm | 5.0 mm |
| M8 | 1.25 | 6.78 mm | 6.8 mm |
| M10 | 1.5 | 8.54 mm | 8.5 mm |
| M12 | 1.75 | 10.29 mm | 10.2 mm |
| M16 | 2.0 | 14.05 mm | 14.0 mm |
| M20 | 2.5 | 17.56 mm | 17.5 mm |
Tips
- Use cutting oil to reduce tap breakage
- Start with a center drill to prevent wandering
- For blind holes, drill at least 1.5× the tap diameter deeper than needed thread depth
- Back the tap out frequently (every 1–2 turns) to clear chips
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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