How to Choose a Tap Drill Size
A practical guide to metric and imperial tap-drill selection, thread percentage, tap type, material and final chart verification.
The correct tap drill depends on the thread system, pitch or TPI, target thread percentage, material, tap style and hole condition. A simple relationship gives a useful starting point, but production drilling should be checked against the approved thread chart and tap manufacturer data.
Selection map
THREAD CALLOUT │ ├─ Metric → D + P → drill ≈ D − P │ └─ Unified → D + TPI → drill ≈ D − 1/TPI ↓ verify chart / class
Examples
| Thread | Starting relationship | Result |
|---|---|---|
| M6 × 1.0 | 6.00 − 1.00 | ≈5.00 mm |
| M8 × 1.25 | 8.00 − 1.25 | ≈6.75 mm |
| 1/4-20 | 0.250 − 1/20 | ≈0.200 in |
Thread percentage
More thread is not automatically better. Higher engagement can increase tapping torque and breakage risk. The desired percentage depends on the application, material and tap process.
Other factors
- Cutting tap vs form tap.
- Through hole vs blind hole.
- Material strength and ductility.
- Lubrication/coolant.
- Hole depth and chamfer.
- Required thread class or fit.
ISO 724:2023 specifies basic dimensions for ISO general-purpose metric threads, while ISO 262:2023 specifies selected metric fastening sizes. ISO 724:2023 · ISO 262:2023.
Tap Drill Size Calculator · Thread Size Converter
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