Calculation method
For the supported 60° thread forms, theoretical drill diameter = major diameter − 1.299038 × pitch × target engagement. The result is then compared with nearby nominal stock drills.
Thread & Tap Drill Calculator
Select a metric coarse/fine or Unified UNC/UNF/UNEF thread, calculate a theoretical 60°-thread tap-drill diameter, and compare it with the nearest source-audited stock drill.
Part of Workshop Calc ↗Ready to calculate
Enter the dimensions, then calculate to see a checkable result.
Estimate only:Confirm the applicable thread standard, tap maker guidance, fit class, and process before cutting a thread.
02 / Method & limits
Make the calculation checkable.
Example
M8 × 1.25 at 75% engagement gives about 6.782 mm; the nearest supported stock drill is 6.8 mm.
Limits & checks
- This release supports ISO metric M-profile and Unified UNC/UNF/UNEF 60° threads; pipe and tapered threads are intentionally excluded.
- Recommended drill size depends on material, tap style, engagement, tolerance class, and process.
Sources & review
- ASME B1.13M — metric M-profile screw threads ↗
- ASME B1.1 — Unified inch screw threads ↗
- ISO 262:2023 — selected general-purpose metric thread sizes ↗
- Guhring Tap Drill Calculator — percentage-of-thread guidance ↗
- NBS Handbook H28 supplement (1957), Part I — 60° basic thread-profile geometry ↗
- ASME B94.11M — twist-drill sizes, nomenclature, and tolerances ↗
- Guhring Drill Size — metric, fractional, number, and letter decimal equivalents ↗
- U.S. National Bureau of Standards CS24-43 — historical screw-thread and tap-drill tables ↗
Last reviewed: 2026-07-19