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Percent of thread, explained

Percent of thread compares the thread depth a tap actually cuts with the full theoretical depth. Tap drill charts usually aim for 75%.

The formula

% thread = TPI × (major diameter − drill diameter) ÷ 0.01299 (inches). For metric threads, the shop rule drill = diameter − pitch gives the standard tap drill.

Why not 100%?

Going from 75% to 100% thread adds only about 5% more strength, but it roughly triples the torque needed to turn the tap. That's how taps break. In most materials a threaded joint is stronger than the bolt once the engagement length is about 1–1.5 × the diameter.

When to drill larger (lower %)

• Stainless steel, titanium and other tough alloys: 60–65%.
• Deep holes, where engagement is more than 1.5 × the diameter.
• Small taps (#0–#6) by hand.
• Thread-forming (roll) taps: these need a much larger drill. Use the maker's chart, not this one.

Each inch tap page lists every drill between about 50% and 85% thread, so you can choose.