Bolt Identifier — Identify Any Bolt by Size, Thread & Grade
Identify any bolt, screw, or fastener by its head markings, diameter, thread pitch, and drive type. This visual guide walks you through measuring a fastener by hand and matching it to the right wrench size, grade, and replacement part. For instant photo-based identification, try the AI Bolt Identifier.
How to identify a bolt by hand
Measure the across-flats of the head with a caliper or wrench to find the wrench size, measure the shank diameter for the bolt size, and count threads over one inch (or use a pitch gauge) for the thread pitch. Note the head type (hex, Torx, Allen, 12-point) and any grade markings stamped on the head.
Reading grade markings
SAE bolts show radial lines on the head: no lines is Grade 2, three lines is Grade 5, six lines is Grade 8. Metric bolts are stamped with a property class number such as 8.8, 10.9, or 12.9. Higher numbers mean stronger, harder steel rated for higher torque.
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