Tap Drill Size Calculator

Work out tap drill size instantly with clear inputs, formula shown and shareable results.

About Tap Drill Size Calculator

Tap drill size trades thread depth against tapping torque: drill diameter equals major diameter minus the engagement fraction times 1.299 divided by threads per inch. Seventy-five percent engagement is the shop standard because it retains about 95 percent of the thread strength while cutting tapping torque roughly in half compared with full depth.

Formulas

Tap drill

Drill = major diameter - (percent engagement x 1.299 / tpi)

Minor diameter

Basic minor = major - 1.299 x pitch

Worked example

With the default values below, Tap Drill Size Calculator returns Tap drill diameter of 0.2013. Change any input above and every figure here is recalculated in your browser.

Inputs used

Thread major diameterminimum 0.02 in: 0.25 in
Threads per inchminimum 1: 20
Thread engagement targetaccepts 40–100 %: 75 %

Results

Tap drill diameter: 0.2013 in
Tap drill in millimetres: 5.113 mm
Thread pitch: 0.05000 in
Basic minor diameter: 0.1851 in

Step by step

1Values used
2Tap drill
3Minor diameter
4Tap drill diameter= 0.2013 in
5Tap drill in millimetres= 5.113 mm
6Thread pitch= 0.05000 in
7Basic minor diameter= 0.1851 in

Assumptions and limits

The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 5 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is the Tap Drill Size figure worked out?

Drill = major diameter - (percent engagement x 1.299 / tpi). Tap drill size trades thread depth against tapping torque: drill diameter equals major diameter minus the engagement fraction times 1.299 divided by threads per inch.

When should I use less than 75 percent?

In tough or work-hardening materials, or for deep holes in blind bores, 60-65 percent reduces tap breakage substantially at little cost in strength.

What values do I need to enter?

This calculator takes 3 inputs: Thread major diameter, Threads per inch, Thread engagement target. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.