Screw and Bolt Size Chart Calculator

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. Coarse 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): the hole the bolt passes through freely, so its diameter is larger than the bolt’s outside diameter. Close clearance runs about 5% over the bolt diameter. Normal clearance is easier to think about as an absolute gap than a percentage: it is 0.4 mm on an M2 and 3 mm on an M30, which works out anywhere from 20% down to 8% as the bolt grows. Tap hole (threaded): drilled before the threads are cut, so its diameter is smaller than the bolt’s. Tap drill size = nominal diameter − thread pitch (approximately). For M8×1.25: tap drill ≈ 8 − 1.25 = 6.75 mm → use a 6.8 mm drill.

Metric and imperial are not interchangeable The one thing to watch when cross-shopping: the stocked metric series runs M10, M12, M14, M16, M20, M24, M30, so there is nothing at all between M10 and M12, and the in-between sizes higher up (M18, M22, M27) are second-choice numbers you will not find on a shop shelf. A 7/16" bolt is 11.11 mm and a 1/2" is 12.7 mm, and both land in that first gap. The nearest real metric size is 6 to 7% away in each case, which is enough to jam in a drilled hole or to rattle in it. This page names the gap rather than rounding to an M11 that nobody makes.

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), used 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, sits flush with the surface. Pan head: rounded top, flat bottom, the most common machine screw. Hex head: tightened with a wrench, used for structural connections. Socket cap (Allen): recessed hex drive, high strength in a compact head. Phillips vs. Torx: Torx (star drive) provides better torque transfer and less cam-out.