Drill Tap Size Calculator

Pick a tap size to get the right drill, with the exact thread engagement for the closest number, letter, fractional and metric drills.

Thread
75% is standard. 50–65% for hard metals and deep holes.
Closest Drills
DrillSizeThread %
▶ marks the closest to your target.
Fractions and decimals both work.
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Tap Drill
Ideal drill diameter
Best drill
Major diameter
Minor diameter (100%)
Pitch
Metric Choice

Tap Drill Chart

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HoleTapDrillThread %Remove

How to Find the Tap Drill Size

A tap drill leaves enough metal for the tap to cut threads into. Drilling at the thread's minor diameter would give a full 100% thread, but that's very hard to tap and barely stronger than about 75%, which is the standard target.

Inch: tap drill = major diameter − 0.01299 × % thread ÷ threads per inch
Metric: tap drill ≈ major diameter − pitch (about 77% thread)
Thread % from a drill = (major diameter − drill) × TPI ÷ 0.01299

For metric threads, the same formula works with TPI = 25.4 ÷ pitch. Numbered machine screw sizes have a major diameter of 0.060″ + 0.013″ × the screw number.

Worked Example

Common Tap Drills at About 75%

TapDrillSizeThread %

Number and letter drill sizes from the standard gauge table (ASME B94.11M).

Tips

Use a slightly larger drill (lower thread %) for stainless, titanium and deep blind holes to cut tap breakage. Roll-form (thread-forming) taps need a larger hole than cutting taps; use the tap maker's chart for those. Chamfer the hole so the first thread doesn't burr.