Thread Major Diameter

Thread major diameter is the largest diameter of a threaded fastener, measured from the crest of one thread to the crest on the opposite side. For external threads (like bolts and screws), it’s essentially the “outside” diameter of the threads and is the primary dimension used to identify thread size. In metric fasteners, the nominal size (the “M” number) corresponds to the major diameter in millimeters, so an M10 fastener has a major diameter of about 10 mm. In Unified inch threads, the major diameter is tied to the nominal size (for example, 1/4 in, 3/8 in), with exact values defined by the thread standard.

Major diameter matters because it affects fit, strength, and compatibility with mating parts. If the major diameter is too large, the fastener may not start into a nut or tapped hole; if it’s too small, the threads may have reduced engagement and lower strength. Along with pitch, minor diameter, and thread class/tolerance, the major diameter helps define how tightly or loosely parts will assemble and how well they will carry load. When measuring, a caliper across the thread crests provides an estimate of major diameter, but precise verification is typically done with thread gages or by referencing the applicable standard dimensions.