How do I calculate the tap drill size for a metric thread?

The tap drill diameter is calculated as: Tap Drill = Nominal Diameter − Pitch. For example, for an M8×1.25 thread: 8 − 1.25 = 6.75mm tap drill. This gives approximately 75% thread engagement, which is the standard for most engineering applications.

What tap drill size do I need for an M6 thread?

For an M6×1.0 coarse pitch thread, the tap drill size is 5.0mm. The clearance drill (for the through-hole in the mating part) is 6.4mm.

What tap drill size do I need for an M10 thread?

For an M10×1.5 coarse pitch thread, the tap drill size is 8.5mm. The clearance drill is 10.5mm.

How long should thread engagement be for a metric thread?

Thread engagement length depends on the material. For steel, a minimum of 1.0× the thread diameter is required, with 1.5× recommended. For aluminium, minimum 1.5× with 2.0× recommended. For plastics, minimum 2.0× with 3.0× recommended, due to the lower shear strength of the material.

Back to ResourcesEngineering Reference

Metric Thread Data

ISO metric thread sizes from M2 to M24.

Metric threads are defined by ISO 68-1 and ISO 261.

How Tap Drill Size is Calculated

The tap drill diameter determines how much material the tap has to cut.

Thread Engagement Length

Thread engagement length is as important as the thread size itself.

Material	Minimum engagement	Recommended
Steel	1.0 × diameter	1.5 × diameter

Coarse Pitch Series

Size	Pitch (mm)	Tap drill (mm)	Clearance drill (mm)
M2	0.4	1.6	2.2

Fine Pitch Series

Fine pitch threads have a smaller pitch for the same nominal diameter.

Size	Pitch (mm)	Tap drill (mm)	Clearance drill (mm)
M6	0.75	5.25	6.4

Notes

Tap drill sizes are calculated for 75% thread engagement.

Request a free review Back to resources