Find a practical starting hole size before cutting a metric internal thread.
Recommended starting drill diameter8.50 mm
For M10 × 1.5 mm, using nominal diameter − pitch.
This is a practical reference for common tapping work. Thread percentage, material, tap type and the manufacturer’s chart may change the preferred size.
How the calculation works
The quick approximation is tapping drill diameter = nominal thread diameter − thread pitch.
For example, M10 × 1.5: 10.0 − 1.5 = 8.5 mm.
Practical tapping checklist
Secure the workpiece and keep the drill perpendicular.
Use the correct drill and cutting fluid for the material.
Break chips regularly when hand-tapping, especially in blind holes.
Do not force a tap that is misaligned or packed with chips.
Wear eye protection and remove chips with a brush.