Sources and scope for this page: ISO 68-1:2023, covering the definition of minor diameter. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
External and internal threads have different minor diameters for the same nominal size, because the external thread has a rounded root and the internal thread a flat crest. Quoting one figure for both is a common error.
The minor diameter is what determines the bolt's tensile stress area and therefore its strength, which is why a fine thread — with a shallower thread depth — leaves a larger minor diameter and makes a stronger bolt at the same nominal size.
Common misuse
Using the external minor diameter for an internal thread (or vice versa). They differ by roughly 0.08 × pitch, which is enough to fail a gauge check.