Pitch Diameter Calculator

Pitch diameter is the diameter of an imaginary cylinder that passes through the thread profile at the point where the thread width equals the space width. For ISO metric threads, the basic pitch diameter is calculated as D₂ = d - 0.649519 × P, where d is nominal diameter and P is pitch. It determines the fit quality between mating threads.

The major diameter is the largest diameter of the thread (crest for external, root for internal). The minor diameter is the smallest diameter (root for external, crest for internal). The pitch diameter lies between them and is the most important for thread fit — it controls whether threads will assemble properly and carry load evenly.

The tolerance class defines the allowable deviation from the basic thread profile. For ISO metric threads: "6g" is standard for external threads (bolts) with a tolerance allowance below basic size. "6H" is standard for internal threads (nuts) with tolerance above basic size. The number indicates tolerance grade (lower = tighter) and the letter indicates position relative to basic size.

Measure the distance between adjacent thread crests in millimeters using a thread pitch gauge. For coarse ISO threads, common pitches are: M6×1.0, M8×1.25, M10×1.5, M12×1.75, M16×2.0, M20×2.5. Fine threads use smaller pitches: M8×1.0, M10×1.25, M12×1.25, M16×1.5. The pitch may also be stamped on the bolt head.