Metric Thread Size Chart (ISO 261 / ISO 965)

Classification: Standard dimensions (calculated from a defined profile)

Major diameters and pitches are standardised values from ISO 261. Pitch diameter (d2) and minor diameter (d1) are calculated from the defined 60° basic profile of ISO 68-1 using d2 = d − 0.6495p and d1 = d − 1.0825p. Tap drill sizes are practical shop recommendations for roughly 75% thread engagement and are not part of the ISO dimension standard.

The basic profile is a 60° symmetric thread form with a flat crest and root. Pitch and minor diameters follow algebraically from the major diameter and pitch. Tap drill sizes use the common shop rule drill ≈ d − p, which yields approximately 75% thread height — a compromise between thread strength and the torque needed to cut the thread.

Classification: Stable (mature standard)

ISO metric thread geometry has been stable for decades and the listed sizes are unchanged across recent standard revisions. Confirm the tolerance class (e.g. 6g/6H) against the current ISO 965 edition for fit-critical work.

• Tabulated d2 and d1 are basic (zero-deviation) values; real threads include tolerances per ISO 965 that shift these dimensions by the class allowance.
• Tap drill sizes assume about 75% engagement in steel. Softer materials (aluminium, brass) may take a slightly smaller drill; harder alloys a slightly larger one.
• The fine-pitch rows list the most common fine pitches, not every fine pitch available in ISO 261.

• For design calculations use the pitch diameter (d2) as the controlling dimension for gauging and tensile stress area.
• Always specify both diameter and pitch (e.g. M10×1.5) — “M10” alone defaults to coarse and is ambiguous in fine-pitch contexts.
• Pair this chart with the tolerance classes from ISO 965 when ordering or inspecting threaded parts.

All linear dimensions are in millimetres. Pitch is the axial distance between adjacent thread crests, not threads-per-inch — use the UNC and UNF charts for inch fasteners.