1/2-Inch Tap Drill Size: 27/64 for 13 TPI, 29/64 for 20 TPI

Use a 27/64-inch drill bit for a conventional 1/2-13 UNC cutting tap. For a 1/2-20 UNF cutting tap, use 29/64 inch. Both pairings appear in the IRWIN HANSON tap-and-drill selection chart.

“1/2 inch” alone is not enough to choose the bit: read the full thread marking and confirm whether the tap cuts or forms threads.

Match the bit to the tap marking

Conventional cutting tap	Thread type	Drill bit	Decimal diameter	Metric equivalent*
1/2-13 UNC, sometimes marked NC	Coarse screw thread, 13 threads per inch	27/64 in	0.421875 in	10.716 mm
1/2-20 UNF, sometimes marked NF	Fine screw thread, 20 threads per inch	29/64 in	0.453125 in	11.509 mm

Metric values are calculated conversions, rounded to three decimal places—not recommendations to substitute a nearby metric bit.

Bolt Depot’s tap-drill table independently lists these same fractional sizes and describes its recommendations as being for 75% thread depth. That percentage describes the thread profile, not how far down the hole you tap.

For these screw threads, the first number identifies nominal thread diameter; the second gives threads per inch. The finer 20-TPI thread uses a larger starting hole than the 13-TPI thread.

Do not substitute a 1/2-inch or 7/16-inch bit by default

A 1/2-inch drilled hole is too large for either of these cutting-tap recommendations: it removes the material needed to create the intended internal thread profile. A hole that lets a bolt pass through is a clearance hole, not a tap hole.

A 7/16-inch bit is not the standard choice for either pitch:

• For 1/2-13, it is 1/64 inch larger than the recommended 27/64-inch bit, leaving less material for the thread.
• For 1/2-20, it is 1/64 inch smaller than the recommended 29/64-inch bit, leaving more material for the tap to remove.

Choose the listed bit rather than the nearest size already in your set. If the exact tap’s instructions specify a different hole for your material or application, follow those instructions.

Two exceptions: pipe taps and forming taps

A 1/2-14 NPT pipe tap needs a separate lookup. Do not use either screw-thread size above. Newman Tools lists 23/32 inch, while the IRWIN chart lists 45/64 inch. Check the exact pipe tap’s drilling instructions rather than treating either chart as universal. Our 1/2-14 NPT drill-size guide covers that distinction.

A forming or roll tap also needs different hole data. It displaces material instead of cutting it away. Guhring explains that forming taps require larger starting holes, with diameter depending on material, workpiece geometry and required thread depth. Do not use the cutting-tap table for a forming tap.

Before drilling, verify the complete marking, tap type and required hole depth. For a blind hole, allow room for the tap’s chamfered lead rather than assuming drilled depth equals usable full-thread depth; IRWIN distinguishes taper, plug and bottoming chamfers for different threading needs.