Hex Bolt Sizes Guide — M4 to M20 Dimensions and Applications

Understanding hex bolt sizing requires knowing the key dimensions. Nominal diameter (M4, M6, M10, etc.) is the nominal thread outer diameter. Thread pitch is the distance between threads — standard metric coarse thread is the default (M8x1.25, M10x1.5, etc.). Bolt length is measured from under the hex head to the tip for full-thread bolts; for half-thread bolts (grip bolts), the grip length (unthreaded shank) is specified separately. Across-flats (AF) is the wrench size needed — M10 uses a 17mm spanner, M12 uses 19mm, M16 uses 24mm. Head height is typically 0.6-0.7x the nominal diameter. At ScrewBazar, hex bolts follow ISO 4014 and ISO 4017 standards for dimensions.

M4 (thread pitch 0.7mm) and M5 (pitch 0.8mm) hex bolts are the smallest practical hex bolt sizes for most light assembly applications. M4 generates approximately 2.8 kN proof load in Grade 8.8 — suitable for electronics enclosures, instrument panels, light sheet metal assembly, and small machinery covers. M5 generates approximately 4.6 kN proof load — used in light machinery, automotive interior components, and appliance assembly. Tools required: 7mm spanner for M4, 8mm spanner for M5. Both sizes are available at ScrewBazar in stainless steel SS304 and zinc-plated mild steel, with matching hex nuts and washers.

M6 (pitch 1.0mm) and M8 (pitch 1.25mm) are the most widely used hex bolt sizes in Indian industrial applications. M6 Grade 8.8 generates approximately 8.0 kN proof load — the workhorse for light machinery, furniture frames, HVAC equipment mounting, and general fabrication. M8 Grade 8.8 generates approximately 14.7 kN — used for heavier machinery mounting, motor flanges, conveyor systems, and structural panel connections. M6 uses a 10mm spanner; M8 uses 13mm. Both sizes are available in the full range of grades (4.6, 8.8, 10.9) and materials (zinc-plated MS, SS304, SS316) at ScrewBazar in large volumes for industrial buyers.

M10 hex bolts (pitch 1.5mm) generate approximately 25 kN proof load in Grade 8.8 — suitable for structural steel connections, machinery base plates, heavy conveyor and material handling equipment, and industrial racking systems. M12 (pitch 1.75mm) generates approximately 36 kN proof load — the standard for structural steelwork in building construction, heavy machinery, and civil infrastructure. M10 requires a 17mm spanner; M12 requires a 19mm spanner. In hot-dip galvanised Grade 8.8, M12 bolts are the standard for Indian construction steel connections under IS 1363. Both sizes are stocked at ScrewBazar in zinc-plated, galvanised, and stainless grades.

M16 hex bolts (pitch 2.0mm) generate approximately 66 kN proof load in Grade 8.8 — used for heavy structural steel connections, column base plates, bridge components, large industrial equipment pads, and foundation anchor bolt assemblies. M20 (pitch 2.5mm) generates approximately 103 kN proof load — the standard for heavy civil engineering, pre-stressed concrete connections, wind turbine foundations, and large industrial structures. M16 requires a 24mm spanner; M20 requires a 30mm spanner. These larger sizes are typically specified by structural engineers with defined torque requirements. ScrewBazar supplies M16 and M20 in Grade 8.8 and 10.9 with material certification for structural applications.

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For every hex bolt, thread engagement must be adequate. When using a nut, the standard hex nut height provides sufficient engagement for Grade 8.8 and below. For Grade 10.9 and 12.9 high-tensile bolts, ensure the nut grade matches — a Grade 10.9 bolt should be paired with a Grade 10 nut. For tapped hole assemblies, minimum engagement depth: 1.0xD in steel, 1.5xD in cast iron, 2.0xD in aluminium. Always use a washer under the bolt head and under the nut to distribute load and protect the surface — ScrewBazar supplies matched sets of hex nuts and washers to match every bolt size.

Across Indian manufacturing and construction, the most commonly used hex bolt sizes in India are M6, M8, M10, and M12 — covering the vast majority of general industrial, machinery, and structural applications. M4 and M5 dominate in electronics and light assembly; M16 and M20 appear in heavy structural and civil engineering work. ScrewBazar maintains deep stock of all sizes from M4 to M20 in zinc-plated and stainless steel, with same-day dispatch for stock sizes. For custom lengths, special grades, or large volume requirements, contact our team for a tailored quote.