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Thread calculator

Basic pitch diameter and three-wire measurement for 60° threads (Unified inch and ISO metric), with worked examples.

Basic pitch diameter

Examples: 1/4-20 → 0.2500 − 0.0325 = 0.2175". M10 × 1.5 → 10 − 0.974 = 9.026 mm. These are basic sizes; the class of fit (2A/2B, 6g/6H and so on) sets the tolerance band below or above them.

Measuring over three wires

The best wire touches the flanks right at the pitch line:

Example, 1/4-20 at basic pitch diameter: W = 0.0289", so M = 0.2175 + 0.0866 − 0.0433 = 0.2608". Use the actual wire size you have in the formula, not the nominal best size.

Thread standards in the app

• Unified inch: UNC, UNF, UNEF, UNS, UNJ, UNM and special UN threads
• ISO metric M profile
• ACME and Stub ACME
• Pipe: NPT and BSPP
• British Whitworth
• STI (helical insert) threads

Each gives major, pitch and minor diameter limits for internal and external threads, plus G76 lathe threading and G84 rigid tapping helpers.