Calculate thread pitch

Convert thread pitch and threads-per-inch, then estimate a tap-drill diameter.

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ToolThread Pitch Calculator
Input
Output
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Preview

How it works

Metric pitch is axial distance per thread, while imperial threads-per-inch invert to millimetres. Tap-drill diameter subtracts a depth factor times pitch and engagement from major diameter.

  • 75% engagement is an editable machining estimate.
  • Metric and imperial inputs remain separate.

Worked example

M10 x 1.5 at 75% engagement
A common metric machine screw thread
Input
											System: metric
Major diameter mm: 10
Pitch mm: 1.5
Output
												Tap drill diameter mm: 8.5386
Tap drill diameter inches: 0.3362
Pitch mm: 1.5
Threads per inch: 16.9333
Major diameter mm: 10
Major diameter inches: 0.3937
Engagement percent: 75

When to use this

Machinists choose M10 drills, fastener inspections convert imperial pitch, and CAD notes record diameter and engagement.

Edge cases

  • 1/4-20 has pitch 1.27 mm, not 20 mm.
  • Tap-drill guidance changes with material, tap type, and tolerance.
  • Nominal diameter does not guarantee mating fit class.

References