Bolt wrench size chart and bolt grades
Pick a bolt size to get the wrench or socket for the head and the nut. Then use the grade chart to read the marks on the bolt head.
Inch bolts (SAE)
| Bolt | Head (cap screw) | Hex nut | Heavy hex nut |
|---|
Metric bolts
| Bolt | Head and nut, ISO | Head and nut, DIN |
|---|
ISO and DIN metric heads differ only at M10 (16 vs 17 mm), M12 (18 vs 19 mm) and M14 (21 vs 22 mm). If a 16 mm socket is loose on an M10 head, try 17.
Bolt grade markings
| Grade / class | Head marking | Min. tensile strength | Typical use |
|---|---|---|---|
| SAE Grade 2 | No marks | 74,000 psi (1/4–3/4 in.) | Light duty, household |
| SAE Grade 5 | 3 radial lines | 120,000 psi (1/4–1 in.) | Automotive, machinery |
| SAE Grade 8 | 6 radial lines | 150,000 psi | High stress: suspension, heavy equipment |
| Metric 4.6 | "4.6" | 400 MPa (58,000 psi) | Light duty |
| Metric 8.8 | "8.8" | 800 MPa (116,000 psi) up to M16 | Similar to Grade 5 |
| Metric 10.9 | "10.9" | 1,040 MPa (151,000 psi) | Similar to Grade 8 |
| Metric 12.9 | "12.9" | 1,220 MPa (177,000 psi) | Socket head cap screws, high stress |
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Questions & answers
What size wrench for a 1/2 in. bolt?
A 1/2 in. hex bolt head takes a 3/4 in. wrench, and so does a standard 1/2 in. hex nut. A 1/2 in. heavy hex nut takes 7/8 in.
What size socket for a 3/8 in. bolt?
9/16 in. for the head and for a standard hex nut. A heavy hex nut on 3/8 in. takes 11/16 in.
What size wrench for an M10 bolt?
16 mm for ISO bolts and nuts, 17 mm for DIN. Both are common, so keep both sockets.
How do I tell a Grade 5 from a Grade 8 bolt?
Count the lines on the head: 3 radial lines is Grade 5, 6 lines is Grade 8, and no lines is Grade 2. Metric bolts are stamped with the class number, such as 8.8 or 10.9.