What thread engagement should I use?

Seventy-five percent is the standard for most work. It captures roughly 95 percent of the joint strength of a full thread while greatly reducing tapping torque and the risk of breaking the tap. Drop to 50 to 60 percent for hard or tough materials.

What is the formula for inch threads?

Tap drill diameter equals the major diameter minus the percent engagement times 1.299 divided by the threads per inch. The 1.299 constant comes from the 60-degree thread geometry of the Unified thread form.

What is the metric formula?

For ISO metric threads the tap drill equals the major diameter minus the percent engagement times the pitch times 1.0825. At 100 percent engagement this reduces to the basic minor diameter of the thread.

Why does higher engagement need a smaller drill?

More thread engagement means more metal left in the hole for the tap to cut, so the starting hole must be smaller. A smaller drill leaves taller threads but also sharply increases the torque the tap must overcome.

Does this account for roll-form taps?

No. These figures are for cutting taps that remove material. Form taps, which displace metal rather than cutting it, require a noticeably larger hole and use a different formula, so use a dedicated form-tap chart for those.

Tap Drill Size Calculator

Drill the tapping hole wrong and you either strip the threads or snap the tap. This calculator gives the correct hole diameter for UNC, UNF, and ISO metric threads at whatever thread engagement you target, and rounds to the nearest standard drill so you can grab the right bit from the index.

How it works

The drill diameter is the major diameter reduced by the amount of thread you want the tap to cut:

inch:   drill = major − (%engagement/100) × (1.299 / TPI)
metric: drill = major − (%engagement/100) × (pitch × 1.0825)

The constants encode the 60-degree thread form. At 100 percent engagement each formula returns the basic minor diameter; the 75 percent default leaves a slightly larger hole that is far easier to tap.

Example and tips

A 1/4-20 UNC thread at 75 percent engagement works out to 0.25 − 0.75 × (1.299/20), or about 0.201 inch — exactly the common #7 tap drill. An M6 × 1.0 thread at the standard engagement comes out near 5.0 mm. In tough stainless or titanium, deliberately lowering engagement to around 60 percent trades a little strength for a big drop in torque, which keeps small taps from shearing off in the hole.