Tap Drill Size Calculator

Before you cut a thread you drill the hole the tap will cut into, and the size of that hole decides everything. Too big and the thread is shallow and strips; too small and the tap binds and snaps off in the work, which is a bad day. This gives you the tap drill for any common metric thread, at the percent of thread you want, plus the clearance hole for the bolt to pass through.

What percent of thread really means

A tap does not fill the hole completely. The percent of thread is how far the threads cut toward a perfect, full form. At 100% the thread is as deep as it can be, but the tap has to remove the most metal and the torque climbs fast. At 75% the thread holds nearly as much load as a full one, perhaps 95% of the strength, for a fraction of the tapping effort. That trade is why 75% is the number machinists reach for without thinking.

The rule hiding in the math

At 75% thread the tap drill works out to the major diameter minus the pitch. That is why an M6, pitch 1 mm, takes a 5 mm drill, and an M8 at pitch 1.25 takes a 6.8. Carry that one rule and you can size most coarse metric taps in your head. This page does the exact geometry so you can dial the percent up or down, but the shortcut is worth knowing.

Softer metal wants a fuller thread, harder wants less

In aluminum, brass or mild steel you can chase 75% or even higher and the tap sails through. In stainless, tool steel or cast iron the tap works hard and snaps easily, so dropping to 60 or 65% thread opens the hole a hair, cuts the torque, and saves the tap with almost no loss of grip. Match the percent to how hard the material fights, not to a chart.

Do not forget the clearance hole

The tap drill is for the piece you are threading. The piece the bolt passes through wants a clearance hole, a touch larger than the bolt so it slides and the head pulls the joint tight. The clearance here is a medium, normal fit per ISO 273. For a tight location pick a size smaller; for rough work where alignment is loose, a size larger makes assembly easy.

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