Tap Drill Size Calculator

Formula

• Metric tap drill ≈ D_major − pitch (75% thread engagement)
• Unified: standard tap drill lookup table

About this calculator

Part of our Machining & CNC Calculators collection. Cutting speed, feeds, RPM, cycle time, and tap drill charts.

How it works

Metric tap drill size equals major diameter minus pitch for roughly 75% thread engagement. Unified threads use standard tap drill lookup values. Pick the closest available drill diameter from your shop set and verify engagement on a test hole.

Enter your values in the inputs above and click CALCULATE. Results appear on the right without a page reload. No login and no server upload.

Worked example

Metric M8 × 1.25 pitch.

1. Tap drill = 8 − 1.25 = 6.75 mm
2. Nominal thread engagement ≈ 75%

Result: Use 6.8 mm (or 6.75 mm) drill for M8 × 1.25.

When to use

• Picking a tap drill before threading a hole
• Checking metric drill size against ISO coarse pitch
• Looking up common UNC/UNF tap drill sizes

Limitations

• 75% engagement is nominal; actual engagement depends on drill oversize and tap type
• Does not cover pipe threads or fine-pitch specials
• Unified table uses common imperial drill sizes in mm

FAQ

Why major diameter minus pitch?

For 60° metric threads, D − pitch gives approximately 75% of thread height engagement with a sharp tap in a drilled hole.

Can I use 65% or 80% engagement?

For harder materials or blind holes, smaller drills (more engagement) are common. This tool uses the standard 75% rule.

What about roll taps or forming taps?

Forming taps need larger pre-drill sizes. This calculator is for cutting taps only.