Tap Drill & Thread Size Calculator
Tap drill size, clearance hole and thread engagement for metric and unified threads.
Basic profile geometry. It assumes a sharp tap in a material that cuts cleanly; ductile metals close in behind the tap and want the drill a few hundredths larger.
How it works
A tap drill is not the minor diameter of the thread. It is deliberately larger, because cutting a full-depth thread gains you almost nothing and costs a great deal.
The relationship is drill = major diameter − (engagement% ÷ 100) × 1.299 × pitch, where 1.299 comes from the truncation of the 60° ISO and Unified profile. At 75% engagement an M6 × 1 lands on 5.0 mm and a 1/4-20 lands on 0.201 in — a #7 drill — which is exactly what the wall charts say, because that is where the charts came from.
The reason 75% is the default everywhere is that thread strength does not scale with engagement. Going from 75% to 100% adds roughly 5% to the stripping strength of the joint and roughly doubles the torque on the tap. In anything harder than aluminium the tap breaks first, and a broken tap in a finished part is a much worse afternoon than a slightly weaker thread.
A tapped joint rarely fails by stripping anyway. With the usual rule of thumb — engagement depth of one diameter in steel, one and a half in aluminium — the bolt breaks before the thread pulls out.
Tap drill for a specific thread
Common questions
Why is the tap drill bigger than the minor diameter?
Because a full-depth thread is not worth cutting. The last 25% of engagement adds about 5% to the strength and roughly doubles the tapping torque. Every workshop chart is built at 75% for that reason.
What clearance hole do I want?
Normal, unless you have a reason. Those are the ISO 273 grades: an M6 gets 6.4 close, 6.6 normal, 7.0 loose. Close fits demand accurately positioned holes — with six bolts on a flange the errors accumulate and something will not line up.
How deep should the thread be?
One diameter of engagement in steel, one and a half in aluminium or cast iron, two in plastic. Deeper gains nothing measurable — the first few threads carry most of the load however long the hole is.
Can I use this for a tapered pipe thread?
No. NPT and BSPT taper, so the drill size depends on how far in the tap goes. They have their own tables.