Tap Drill Size Calculator
Find the drill size for a tapped hole, in metric and imperial threads. Sizes are set for roughly 75% thread engagement — the usual balance of strength and tapping effort.
Select a thread to see the tap drill size.
How It Works
Metric tap drills equal the major diameter minus one thread pitch. Imperial sizes use the standard published drill for ~75% thread engagement. For shallow or blind holes, consider 50% engagement (a slightly larger drill) to reduce tapping torque and breakage.
Printable PDF
Download the Tap Drill Size Calculator and its formula sheet as a free PDF.
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Reference values only — always confirm critical fastening decisions against your engineering specification or the fastener manufacturer.