Bolt Size Chart (Inch & Metric, with Wrench Sizes)

Inch bolt size chart (UNC and UNF)

Head sizes are ASME B18.2.1 hex bolts and cap screws; nut sizes are ASME B18.2.2. Bold marks the sizes where the bolt head and the nut take different wrenches: a 7/16 in bolt has a 5/8 in head but an 11/16 in nut.

¹ We found only one source for the 9/16 in hex cap screw head, so we've left it blank rather than guess.

Metric bolt size chart (ISO)

ISO 272 (1982) made the M10, M12 and M14 heads one size smaller than the old DIN bolts. Both are still sold, so an M10 bolt can need a 16 mm or a 17 mm wrench. A metric size with no pitch given (just "M10") means the coarse pitch.

Bolt grade markings

For metric classes, the first number x 100 is roughly the minimum tensile strength in MPa, and the second number x 10 is the yield strength as a percent of tensile. Replace a bolt with the same grade the maker used, and never a lower one.

About torque: we don't list torque values here. The right torque depends on the grade, the thread condition and lubrication, and the joint. Portland Bolt notes that torque alone can't guarantee proper bolt tension. Use the spec from the equipment or vehicle maker.

How to measure a bolt

1. Diameter: calipers across the threads. Match it to the tables above.
2. Pitch: a thread gauge. Without one, count threads over 1/2 in and double it (inch), or measure across 10 threads and divide by 10 (mm).
3. Length: from under the head to the tip for hex bolts. Flat (countersunk) heads are measured overall.
4. Head: fit wrenches until one is snug. The wrench size points you to the bolt size, but check the bold rows above.

Step-by-step with examples is on what size bolt do I need.

Common mistakes

• Assuming nut and bolt take the same wrench. A 7/16 in nut takes 11/16 in but the bolt head takes 5/8 in. A 9/16 in nut takes 7/8 in.
• Fighting a 16 vs 17 mm head. M10 bolts come both ways (ISO vs DIN). Same for 18/19 mm on M12 and 21/22 mm on M14.
• Using a standard wrench on heavy hex. Heavy hex bolts and nuts are 1/8 in larger across the flats (1/2 in heavy hex = 7/8 in).
• Mixing inch and metric look-alikes. 5/16 vs M8, 3/8 vs M10, 5/8 vs M16. Close in size, never interchangeable.
• Swapping in an unmarked bolt. If the original had 3 or 6 lines (or 8.8/10.9), the replacement needs the same.

Not sure what bolt you have? Snap it

Snap the old bolt next to a ruler and Mechy tells you the size, thread and length. If the thread has to be exact, confirm it with a thread gauge or a nut that fits. Identify a bolt from a photo.