Tap Drill Size Calculator – Unified (UN) – Actual

NOTE: This is a specialized calculator, and it’s probably NOT the calculator you are looking for. This calculator generates drill size for an actual percentage of the UN thread form.

Warning: This is NOT the conventional way that tap drills are calculated. See the conventional method here.

This calculator uses the equation from Machinery’s Handbook, 27th edition, page 1934, but this is NOT HOW THE TABLES ARE HANDBOOK TABLES CALCULATED. The drills calculated with this method will produce shorter (weaker) threads than the conventional method.

Major Diameter: inches

Percent Full Thread: Enter as Percent (90 for 90%)

Threads Per Inch: TPI

Tap Drill Size: inches

Schematic of what this calculator generates.

This calculator uses the following formula:

Tap drill formula used in this calculator.


Machinery’s Handbook, 27th ed. Page 1934

See Also: Unified Screw Threads