Tap Drill Size for 5/8-18 UNF

Thread Specifications for 5/8-18 UNF

Here are the complete specifications for the 5/8-18 UNF thread:

• Major Diameter: 0.625″
• Threads Per Inch: 18
• Tap Drill Size: 37/64 (0.5781″ / 14.68mm)
• 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 5/8-18 UNF:

Tap Drill = 0.625″ – (1 / 18) = 0.625″ – 0.0556″ = 0.5694″

The nearest standard drill size to 0.5694″ is 37/64 (0.5781″), 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 37/64 drill provides for 5/8-18 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 1/2-20 UNF — 29/64 (0.4531″)
• Tap Drill for 9/16-18 UNF — 33/64 (0.5156″)
• Tap Drill for 3/4-16 UNF — 11/16 (0.6875″)
• Tap Drill for 7/8-14 UNF — 13/16 (0.8125″)