Tap Drill Size for 3/4-16 UNF

Thread Specifications for 3/4-16 UNF

Here are the complete specifications for the 3/4-16 UNF thread:

• Major Diameter: 0.75″
• Threads Per Inch: 16
• Tap Drill Size: 11/16 (0.6875″ / 17.46mm)
• 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 3/4-16 UNF:

Tap Drill = 0.75″ – (1 / 16) = 0.75″ – 0.0625″ = 0.6875″

The nearest standard drill size to 0.6875″ is 11/16 (0.6875″), 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 11/16 drill provides for 3/4-16 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 9/16-18 UNF — 33/64 (0.5156″)
• Tap Drill for 5/8-18 UNF — 37/64 (0.5781″)
• Tap Drill for 7/8-14 UNF — 13/16 (0.8125″)
• Tap Drill for 1-12 UNF — 15/16 (0.9375″)