Tap Drill Size for #12-24 UNC

Thread Specifications for #12-24 UNC

Here are the complete specifications for the #12-24 UNC thread:

• Major Diameter: 0.216″
• Threads Per Inch: 24
• Tap Drill Size: 16 (0.177″ / 4.5mm)
• Thread Engagement: ~75% (standard)

How Is the Tap Drill Size Calculated?

The tap drill size for unified threads is calculated using the basic formula:

Tap Drill Diameter = Major Diameter – (1 / TPI)

For #12-24 UNC:

Tap Drill = 0.216″ – (1 / 24) = 0.216″ – 0.0417″ = 0.1743″

The nearest standard drill size to 0.1743″ is 16 (0.177″), which gives approximately 75% thread engagement — the recommended percentage for most applications.

Why 75% Thread Engagement?

Thread engagement percentage determines how deep the threads are cut into the material. While it might seem like more is better, there’s a practical limit:

• 50% engagement provides about 90% of the thread’s full strength — use a slightly larger drill for softer materials or where tap breakage is a concern.
• 75% engagement is the industry standard — provides excellent strength with reasonable tapping effort. This is what the 16 drill gives you for #12-24 UNC.
• Beyond 83% engagement adds almost no additional strength but significantly increases tapping torque and the risk of breaking the tap.

Engineering tip: In aluminum, brass, or other soft materials, 50-60% thread engagement is often preferred. Save the 75%+ for steel and other hard materials where you need maximum holding force.

Related Thread Sizes

• Tap Drill for #8-32 UNC — 29 (0.136″)
• Tap Drill for #10-24 UNC — 25 (0.1495″)
• Tap Drill for 1/4-20 UNC — #7 (0.201″)
• Tap Drill for 5/16-18 UNC — F (0.257″)