Pick Metric or Unified, enter the major diameter and pitch (or threads per inch), and this returns the pitch diameter, minor diameter, and tap drill diameter for your target thread engagement — all in one place.
Thread inputs
Only the pitch field is used in Metric mode, only the TPI field is used in Unified mode — the other is ignored. 75% engagement is the standard shop default.
Round the tap drill diameter up to the nearest stocked drill size for a slightly looser/easier-tapping fit, or down for a tighter fit closer to full engagement.
How it works
Both thread systems use the same 60° V-thread geometry, so one formula set covers both once pitch is in consistent units. Pitch diameter = major diameter − 0.6495 × pitch. Minor diameter (100% thread depth) = major diameter − 1.0825 × pitch. Tap drill diameter = major diameter − (engagement % ÷ 100) × 1.0825 × pitch — at 100% engagement this equals the minor diameter exactly, and at the standard 75% it leaves enough material for a strong thread without excess tapping torque. For Unified threads, pitch is simply 1 ÷ threads per inch.
FAQ
Why not always tap at 100% engagement? It needs more torque, breaks taps more often, and barely adds strength over 75% — most shops standardize on 65–75% for general work.
Can I use this for internal AND external threads? Pitch diameter and minor diameter apply to both. Tap drill diameter is for the internal (nut/hole) side only — for external threads, major and minor diameter are cut directly to size.
My TPI doesn’t give a clean pitch — is that normal? Yes, inch pitches are fractions (e.g. 1/16 = 0.0625in) and rarely round numbers in decimal form.
Related Guides
G-Code & CNC Calculators · Dimensions & Geometry Calculators · Setup & Workholding Calculators