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Tap Drill Size Calculator

Calculate the correct tap drill diameter for Unified (UNC/UNF) threads at any desired thread engagement percentage.

Thread major (outer) diameter in inches. E.g. 1/4-20 = 0.25".
Number of threads per inch. E.g. 1/4-20 has 20 TPI. 1/4-28 (UNF) has 28 TPI.
Desired thread engagement percentage. 75% is standard for steel; 50–65% for brittle or soft materials.

Results

Tap Drill Size0.2,013"
Tap Drill Size (mm)5.11 mm

📖What is it?

The tap drill size is the hole diameter drilled before tapping threads. It must be large enough to allow tapping without breaking the tap, yet small enough to provide adequate thread engagement for the required pull-out strength. The standard 75% engagement balances tool life against thread strength.

🎯How to use

Enter the thread major diameter in inches (e.g. 0.25 for 1/4"), the TPI (e.g. 20 for 1/4-20 UNC), and the desired thread engagement percentage. The calculator outputs the recommended tap drill size in both inches and millimetres. Select the nearest standard drill size from a drill index.

💡Example scenario

1/4-20 UNC thread at 75% engagement: Drill = 0.25 − 0.75 × (1.299/20) = 0.25 − 0.04871 = 0.2013" ≈ #7 drill (0.201"). The standard #7 drill at 0.201" is the textbook tap drill for 1/4-20.

🏆Pro tip

Reduce thread engagement to 50–65% when tapping brittle materials (cast iron, brass) or when breaking taps is a concern. A lower engagement drill is easier to tap, breaks fewer taps, and still provides adequate strength — threaded fasteners almost always fail in the shank before a properly engaged thread strips.