Screw and Bolt Size Chart Calculator
Look up screw and bolt dimensions, thread pitch, and drill or tap sizes.
Covers imperial #4 to 3/4 inch and metric M2 to M30 fasteners in one chart.
Imperial Screw Sizes In the United States and Canada, screws smaller than 1/4" use a number designation (#0 through #14). The formula for major diameter: d (in) = 0.060 + 0.013 × screw number So #10 = 0.060 + 0.013 × 10 = 0.190" major diameter. Above #14, sizes switch to fractional inches: 1/4", 5/16", 3/8", 1/2", etc.
Thread Types UNC (Unified National Coarse): standard coarse thread. Best for most applications. UNF (Unified National Fine): tighter thread pitch. Better resistance to vibration, more precise. Coarse threads are more forgiving of dirt, damage, and cross-threading. Fine threads provide more clamping force for same torque.
Metric Fasteners (ISO) Metric bolts use “M” designation followed by nominal diameter in millimeters. M6 = 6 mm diameter. M10 = 10 mm diameter. Standard pitch for M6 = 1.0 mm. Metric ISO standard is used in most of the world outside North America. Course thread pitch (standard): M6×1.0, M8×1.25, M10×1.5, M12×1.75, M16×2.0.
Clearance Hole vs. Tap Hole Clearance hole (through hole): hole that the bolt passes through freely. Diameter > bolt OD. Close clearance: ~bolt diameter + 5%. Normal clearance: + 8–12%. Tap hole (threaded): drilled before tapping threads. Diameter < bolt OD. Tap drill size = nominal diameter − thread pitch (approximately). For M8×1.25: tap drill ≈ 8 − 1.25 = 6.75 mm → use 6.8 mm drill.
Torque Guidelines Tightening torque depends on bolt grade/strength and lubrication. Grade 2 (low strength): common hardware store bolt. Grade 5: medium strength (marked with 3 radial lines on head). Grade 8: high strength (marked with 6 radial lines) — for structural applications. Metric property classes: 8.8 (medium), 10.9 (high), 12.9 (very high strength). Never torque a bolt more than 75% of its yield torque without engineering review.
Screw Head Types Flat head: countersunk — flush with surface. Pan head: rounded top, flat bottom — most common machine screw. Hex head: tightened with a wrench — for structural connections. Socket cap (Allen): recessed hex drive — high strength, compact. Phillips vs. Torx: Torx (star drive) provides better torque transfer and less cam-out.
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