Bolt Size Chart — Metric & SAE Dimensions
Every standard bolt size in one place: metric M4 through M36 and SAE 1/4” through 1-1/2”, with head dimensions, thread pitch, and the exact wrench or socket size for each. Use this chart before any hardware-store run, parts order, or fastener identification job. Printable PDF version available below.
How to read this chart
Each row is a bolt diameter. Metric bolts are identified by M-number (the shank diameter in millimeters); SAE bolts by fractional inches. Thread pitch is the distance between threads — metric pitches are in mm, SAE in threads-per-inch (TPI). The wrench/socket column is the across-flats measurement of the hex head, which determines which tool you need.
Metric bolt sizes (M4–M36)
Metric hex bolts follow ISO 898 (property classes 4.6 through 12.9). Common automotive sizes are M6, M8, M10, M12, and M14. Thread pitch comes in coarse (standard) and fine; coarse is default unless the spec says otherwise. Head sizes are standardized: M8 = 13mm wrench, M10 = 16mm or 17mm, M12 = 18mm or 19mm.
SAE bolt sizes (1/4”–1-1/2”)
SAE hex bolts follow ASTM/SAE grade standards (Grade 2, 5, 8). Common sizes are 1/4”, 5/16”, 3/8”, 7/16”, and 1/2”. Thread counts are UNC (coarse) or UNF (fine). Head markings identify grade: no marks = Grade 2, three radial lines = Grade 5, six radial lines = Grade 8.
When to use metric vs SAE
Virtually every passenger vehicle sold in the US after 1992 uses metric fasteners, including domestic makes. SAE bolts are still common on older domestic trucks, lawn equipment, some appliances, and ham-radio hardware. If your project is post-1995 and not lawn equipment, assume metric.
Related lookups
Loading interactive lookup…