12-24 Tap Drill Size and Drill Bit Guide

Use a #16 drill bit

12-24 · 0.1770 in / 4.50 mm · ~75% thread

12-24 is a small UNC machine screw thread used in electrical fittings, HVAC controls and compact assemblies. A #16 bit is the standard tap drill for a reliable 75% thread.

At a glance

Tap drill	#16
Inches	0.1770 in
Millimetres	4.50 mm
Major diameter	0.2160 in
Pitch	0.0417 in
Threads per inch	24

Why this size

12-24 tap drill: drillØ = major − (pitch × 0.75) = 0.2160 − (0.0417 × 0.75) = 0.1848 in → next standard drill #16 (0.1770 in / 4.50 mm)

Use a #16 for the standard 75% fit. The 12-28 fine variant is rarer and takes a #14 bit (0.1820 in). This reference covers the standard 12-24 coarse thread.