Tap Drill Size for #12-28 UNF

Thread Specifications for #12-28 UNF

Here are the complete specifications for the #12-28 UNF thread:

• Major Diameter: 0.216″
• Threads Per Inch: 28
• Tap Drill Size: 14 (0.182″ / 4.62mm)
• Thread Engagement: ~75% (standard)

How Is the Tap Drill Size Calculated?

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

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

For #12-28 UNF:

Tap Drill = 0.216″ – (1 / 28) = 0.216″ – 0.0357″ = 0.1803″

The nearest standard drill size to 0.1803″ is 14 (0.182″), 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. While more might seem better, there is a practical limit:

• 50% engagement provides about 90% of full thread strength — ideal for softer materials or when tap breakage is a concern.
• 75% engagement is the industry standard — excellent strength with reasonable tapping effort. This is what the 14 drill provides for #12-28 UNF.
• Beyond 83% adds almost no additional strength but significantly increases tapping torque and tap breakage risk.

Pro tip: In aluminum, brass, or other soft materials, 50-60% thread engagement is often preferred to reduce tap breakage. Save 75%+ for steel and harder materials.

Related Thread Sizes

• Tap Drill for #8-36 UNF — 29 (0.136″)
• Tap Drill for #10-32 UNF — 21 (0.159″)
• Tap Drill for 1/4-28 UNF — #3 (0.213″)
• Tap Drill for 5/16-24 UNF — I (0.272″)