ISO 965 is the definitive international standard for ISO General Purpose Metric Screw Thread Tolerances. It establishes the principles and technical data for the tolerance system of metric threads (M series) with the basic profile compliant with ISO 68-1.

This standard is critical for ensuring the interchangeability of fasteners across global supply chains. It defines the limits of sizes for major, minor, and pitch diameters, utilizing a system of tolerance grades (numbers) and tolerance positions (letters) to specify the fit between external threads (bolts) and internal threads (nuts).

The technical focus of ISO 965 is the Thread Fit Class. Tolerances are designated by a combination of a number (Grade) and a letter (Position). For example, 6H for internal threads (nuts) and 6g for external threads (bolts) represent the standard "medium" fit used in general engineering. The letter position indicates the fundamental deviation: uppercase (G, H) for internal and lowercase (e, f, g, h) for external threads.