Tap Drill Size Calculator

measuring TPI

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In our tap drill size calculator, the external diameter, thread count, series number, and thread class tolerances will be used to calculate all results for your tapping operation.

The algorithms in this calculator, including hole size and tap drill sizes, were developed based on the best understanding of the ANSI/ASME B1.1-1989 specification.

Definition of Terms

Imperial UN (Imperial Unified National Thread Form)

A term that uses old units of length as measurements to describe an internal or exterior thread geometry or the thread size geometry.

tap drill sizes

Metric UN (American Metric Unified National Thread Form)

A term that uses metric (mm) dimensions to describe an internal thread or external thread nominal size [1].

Outside Diameter

The spiral part with a major diameter. The largest screw or threaded hole size (hole diameter), whichever is greater.

TPI (Threads per inch)

A measure of a full thread per inch length.

measuring TPI


Space in the middle of each “V” or thread. Basic size includes both the fine series and coarse thread.

Cutting Tap

An equipment that cuts material as it slides along tapped holes to make the winding section of a hole that is threaded. 

Form Tap

As it slides along a pre-drilled hole, a cold-forming tap tool creates the winding section of a hole that is threaded. This tap drill does not have “flutes” and is typically oval.

tap hole

Thread Percentage

The maximum permissible height, the “V” shape, counting flats, is 3/4qtr, or (6/8th), of a 60-degree triangle. 

Robert Johnson is a woodworker who takes joy in sharing his passion for creating to the rest of the world. His brainchild, Sawinery, allowed him to do so as well as connect with other craftsmen and women. He has since built an enviable workshop for himself and an equally impressive online accomplishment: an extensive resource site serving old timers and novices alike.
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