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Drill Tap Size Calculator

For a 60° ISO metric thread the depth of engagement is set by how much smaller the drilled hole is than the major diameter.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the thread's major (nominal) diameter D in mm — the '10' in M10 — and the pitch P in mm (1.5 for standard coarse M10, 1.25 for fine).
  2. Optionally set a target thread engagement %: leave it at 76.8 for the standard 'D − P' hole, or lower it (60–70%) for easier tapping in hard or gummy material.
  3. Read the recommended tap drill diameter, then pick the nearest drill you actually have — round toward a slightly larger drill for lower engagement/easier tapping, or slightly smaller for maximum strength.

How it works

For a 60° ISO metric thread the depth of engagement is set by how much smaller the drilled hole is than the major diameter. The workshop rule of thumb 'tap drill = major diameter − pitch' produces about 76.8% thread engagement, which balances thread strength against tapping torque. This tool generalises that rule: tap drill = D − (engagement% / 76.8) × pitch, so you can trade a little thread depth for much lower tapping torque.

Higher engagement (a smaller hole) gives a stronger thread but sharply higher tapping torque and tap-breakage risk; research shows going much above ~75% adds very little pull-out strength for a lot more torque. The tool also reports the approximate 100%-engagement minor diameter (D − 1.0825 × P) so you can see the theoretical limit. Results are estimates for standard ISO metric threads — always cross-check a reputable tap-drill chart and the tap maker's recommendation for your exact size and material.

Worked example

Tap drill for an M10 × 1.5 thread. For a standard coarse M10 thread the major diameter D = 10 mm and pitch P = 1.5 mm. Leaving engagement at the default 76.8%, the tap drill = 10 − (76.8 / 76.8) × 1.5 = 10 − 1.5 = 8.50 mm. So an 8.5 mm drill gives roughly 77% thread engagement — the usual workshop choice. Dropping the target to 65% (easier tapping in tough steel) gives 10 − (65/76.8) × 1.5 = 8.73 mm, so you would step up toward an 8.8 mm drill.

Common mistakes

  • Confusing pitch with TPI. Metric threads are specified by pitch in mm (distance between crests), not threads per inch. Convert first if you only have a TPI figure.
  • Chasing 100% thread engagement. A near-full-depth hole barely increases strength but hugely increases tapping torque and the chance of snapping the tap — 65–80% is normal.
  • Using this for tapered (NPT/BSPT) pipe threads or UN/imperial threads. The D − P relation here is for straight 60° ISO metric threads only; imperial and pipe threads use different charts.

Frequently asked questions

Why is the standard tap drill just 'diameter minus pitch'?

For a 60° ISO metric thread, drilling the hole to D − P leaves about 76.8% of the theoretical thread depth. That level of engagement gives nearly all the practical pull-out strength of a full thread while keeping tapping torque manageable, so 'D minus pitch' became the default workshop rule and is what this calculator uses when you leave engagement at 76.8%.

Should I use a lower thread engagement percentage?

Often, yes. In hard, tough or abrasive materials (stainless, tool steel) many machinists target 60–70% engagement to cut tapping torque and reduce tap breakage, at almost no cost in joint strength. Lower the engagement % and the tool returns a slightly larger drill. For soft materials or thin sections where every bit of thread matters, keep it near 75–80%.

Does this work for fine-pitch and imperial threads?

It works for any straight metric thread as long as you enter the correct pitch (e.g. M10 × 1.25 fine uses P = 1.25). It does NOT apply to imperial UN threads or tapered pipe threads (NPT/BSPT) — those need their own drill charts because of different geometry and taper.

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