Tap Drill Size Calculator

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In this tap drill size calculator, I incorporated parameters like external diameter, thread count, series number, and thread class tolerances to give you precise 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

In terms of threaded components, I’ve observed that the spiral part has a major diameter. It’s the size of the largest screw or the diameter of the threaded hole, depending on which one 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

Over the years, I’ve used this tool that slices through material as it glides along tapped holes, crafting the helical section of a threaded hole.

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. 

(If you do not want to manually compute your tap drill size, you can check out this chart of tap drill sizes to get a general idea of the right drill size for your screws or bolts.)

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Robert Johnson is a passionate furniture maker & carpenter, sought after for his knowledge on the craft.
You’ve probably seen his down-to-earth wisdom in USA Today, Bobvila, Family Handyman, and The Spruce, where he has shared commentary and guidance on various woodworking topics.

Robert is the brain behind Sawinery, where he aims to share tips, tricks, and a passion for all things carpentry.

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