Tap drill size for metric threads

Result

Tap drill

M8: tap drill 6.8 mm

Thread data

Value	Result
Pitch P	1.25 mm
Tap drill for cutting taps	6.8 mm
Tap drill for forming taps (guide)	7.38 mm
Minor diameter of internal thread D1	6.647 mm
Pitch diameter d2	7.188 mm
Minor diameter of bolt d3	6.466 mm
Thread height H	1.083 mm

Tap drill sizes, coarse threads

Thread	Pitch	Tap drill
M2	0.4 mm	1.6 mm
M2.5	0.45 mm	2.05 mm
M3	0.5 mm	2.5 mm
M4	0.7 mm	3.3 mm
M5	0.8 mm	4.2 mm
M6	1 mm	5 mm
▶ M8	1.25 mm	6.8 mm
M10	1.5 mm	8.5 mm
M12	1.75 mm	10.2 mm
M14	2 mm	12 mm
M16	2 mm	14 mm
M18	2.5 mm	15.5 mm
M20	2.5 mm	17.5 mm
M22	2.5 mm	19.5 mm
M24	3 mm	21 mm
M27	3 mm	24 mm
M30	3.5 mm	26.5 mm
M36	4 mm	32 mm

Notes

• The tap drill for the 8 mm thread with 1.25 mm pitch is 6.8 mm, roughly the nominal diameter minus the pitch.
• Forming taps displace material instead of cutting it and need a slightly larger hole, about nominal diameter minus half the pitch; the tool maker's figure governs.

The values are the usual chart values for cutting taps in normally machinable materials. Material, thread depth and tool can call for a different drill; the markings on the tap come first.