What This Calculator Does
This calculator works out the key dimensions of a standard 60-degree V-form screw thread — pitch diameter, minor diameter, and recommended tap drill size — from just the major (nominal) diameter and threads per inch.
The Formula
The thread pitch is 1/TPI, and the fundamental triangle height is H = 0.866025 x pitch. Pitch diameter is the major diameter minus 0.649519 x pitch, and the theoretical minor diameter is the major diameter minus 1.082532 x pitch. The tap drill size subtracts a fraction of the full thread depth based on your chosen percentage of thread engagement (75% is the common shop default, giving strong threads without excessive tapping torque).
Good to Know
- These formulas apply to standard 60-degree Unified and ISO-style V-threads; specialty thread forms (Acme, buttress, pipe threads) use different geometry.
- Always round the calculated tap drill to the nearest available drill bit size, and verify against a tap drill chart for critical or load-bearing applications.
Last reviewed August 2026