Hex Bolt Dimensions Chart: Complete Metric & Imperial Siz...

Incorrect bolt dimensions are one of the top causes of fastener failure in construction and manufacturing. A bolt that is too short may not develop adequate thread engagement, reducing joint capacity by up to 50%. A bolt head that is too large for the fixture creates interference; too small and the washer face cannot distribute load properly. In procurement, ordering the wrong dimensions wastes 3-8% of project fastener budgets on average through returns and reorders. Understanding hex bolt dimension standards ensures you specify correctly the first time.

Metric hex bolts are designated by thread diameter (M) and length in millimeters. ISO 4014 defines partially threaded hex bolts (the standard for structural applications), while ISO 4017 defines fully threaded hex bolts. The across-flats (AF) dimension determines the wrench size needed. Thread pitch can be coarse (standard) or fine — coarse pitch is the default for most applications and provides better resistance to stripping and contamination.

Imperial (inch) hex bolts follow ASME B18.2.1 in the US market. While metric is the global standard, imperial bolts remain common in North American construction, oil & gas, and legacy infrastructure worldwide — including older installations across Africa and the Middle East. UNC (Unified National Coarse) is the standard thread series, similar to metric coarse pitch.

For ISO 4014 partially threaded hex bolts, thread length depends on bolt diameter. The formula is: bolt length ≤ 125mm → thread length = 2d + 6mm; 125mm < bolt length ≤ 200mm → thread length = 2d + 12mm; bolt length > 200mm → thread length = 2d + 25mm (where d = nominal diameter). The unthreaded shank (grip length) is critical for shear-loaded joints — the shank should pass through both connected parts without threads in the shear plane. For structural joints, specify bolt length so that the grip length matches the total thickness of connected parts.

ISO 4014/4017 specifies tolerances that affect fit and function. Head across-flats tolerance is typically -0 to +0.5mm for sizes up to M24. Body diameter tolerance follows ISO general tolerances (grade g for bolt bodies). Thread tolerance class 6g is standard for general-purpose bolts — this provides a clearance fit with the 6H nut thread. For high-precision applications, specify 4g6g (tighter). Length tolerance for bolt length is typically ±1.5mm for lengths up to 100mm. When ordering, always specify the standard (ISO or ASME) — mixing systems causes fit problems.

The most common procurement errors are: confusing bolt length with thread length (always specify total bolt length per the standard); mixing metric and imperial in the same order (clearly separate line items); not specifying thread pitch for metric bolts above M18 where fine pitch alternatives exist; and ordering partially threaded when fully threaded is needed (or vice versa). For African infrastructure projects, metric is the standard, but always confirm the project specification — some oil & gas projects use imperial. Create a fastener schedule table for each project listing: size, grade, length, thread type, coating, and quantity. For more information, also see our guides on [bolt grades](/en/industry/bolt-grade-markings-guide), [carriage bolts](/en/industry/carriage-bolts-specifications), [nuts and bolts](/en/industry/nylon-nuts-bolts-guide).