Wrench size → bolt diameter guide
Browse SAE wrench across-flats sizes against ASME B18.2.1 regular or heavy hex bolt diameters, UNC/UNF threads when listed, and the nearest ISO metric size — sources cited.
Browse the wrench or socket across-flats sizes and the matching ASME B18.2.1 regular or heavy hex bolt diameter — used when you know the wrench but not the bolt size for ordering. Filter to jump straight to an opening you already know.
Regular hex vs heavy hex
ASME B18.2.1 defines two head styles with different across-flats for the same nominal diameter. Regular hex is common on general fasteners; heavy hex (larger wrench for the same shank) is typical on structural / many Class 7–8 hub and axle fasteners. This guide labels which series each wrench opening belongs to — they do not share the same AF sizes.
| Wrench size (across flats) | Regular hex ASME B18.2.1 | Heavy hex ASME B18.2.1 | UNC thread | UNF thread | Nearest ISO metric |
|---|---|---|---|---|---|
| 1/4″No ASME B18.2.1 hex bolt in this range | — | — | — | — | — |
| 5/16″No ASME B18.2.1 hex bolt in this range | — | — | — | — | — |
| 3/8″No ASME B18.2.1 hex bolt in this range | — | — | — | — | — |
| 7/16″ | 1/4″ | — | 1/4-20 | 1/4-28 | M6(6 mm) |
| 1/2″ | 5/16″ | — | 5/16-18 | 5/16-24 | M8(8 mm) |
| 9/16″ | 3/8″ | — | 3/8-16 | 3/8-24 | M10(10 mm) |
| 5/8″ | 7/16″ | — | 7/16-14 | 7/16-20 | M12(12 mm) |
| 11/16″ | — | 3/8″ | 3/8-16 | 3/8-24 | M10(10 mm) |
| 3/4″ | 1/2″ | — | 1/2-13 | 1/2-20 | M12(12 mm) |
| 13/16″ | 9/16″ | — | 9/16-12 | 9/16-18 | M14(14 mm) |
| 7/8″ | — | 1/2″ | 1/2-13 | 1/2-20 | M12(12 mm) |
| 15/16″ | 5/8″ | — | 5/8-11 | 5/8-18 | M16(16 mm) |
| 1-1/16″ | — | 5/8″ | 5/8-11 | 5/8-18 | M16(16 mm) |
| 1-1/8″ | 3/4″ | — | 3/4-10 | 3/4-16 | M20(20 mm) |
| 1-1/4″ | — | 3/4″ | 3/4-10 | 3/4-16 | M20(20 mm) |
| 1-5/16″ | 7/8″ | — | 7/8-9 | 7/8-14 | M22(22 mm) |
| 1-7/16″ | — | 7/8″ | 7/8-9 | 7/8-14 | M22(22 mm) |
| 1-1/2″ | 1″ | — | 1-8 | 1-12 | M24(24 mm) |
| 1-5/8″ | — | 1″ | 1-8 | 1-12 | M24(24 mm) |
| 1-11/16″ | 1-1/8″ | — | 1-1/8-7 | 1-1/8-12 | M30(30 mm) |
| 1-13/16″ | — | 1-1/8″ | 1-1/8-7 | 1-1/8-12 | M30(30 mm) |
| 1-7/8″ | 1-1/4″ | — | 1-1/4-7 | 1-1/4-12 | M33(33 mm) |
| 2″ | — | 1-1/4″ | 1-1/4-7 | 1-1/4-12 | M33(33 mm) |
| 2-1/16″ | 1-3/8″ | — | 1-3/8-6 | 1-3/8-12 | M36(36 mm) |
| 2-3/16″ | — | 1-3/8″ | 1-3/8-6 | 1-3/8-12 | M36(36 mm) |
| 2-1/4″ | 1-1/2″ | — | 1-1/2-6 | 1-1/2-12 | M39(39 mm) |
| 2-3/8″ | — | 1-1/2″ | 1-1/2-6 | 1-1/2-12 | M39(39 mm) |
| 2-7/16″ | 1-5/8″ | — | — | — | M42(42 mm) |
| 2-9/16″ | — | 1-5/8″ | — | — | M42(42 mm) |
| 2-5/8″ | 1-3/4″ | — | — | — | M45(45 mm) |
| 2-3/4″ | — | 1-3/4″ | — | — | M45(45 mm) |
| 2-13/16″ | 1-7/8″ | — | — | — | M48(48 mm) |
| 2-15/16″ | — | 1-7/8″ | — | — | M48(48 mm) |
| 3″ | 2″ | — | — | — | M52(52 mm) |
| 3-1/8″ | — | 2″ | — | — | M52(52 mm) |
| 3-3/8″ | 2-1/4″ | — | — | — | M56(56 mm) |
| 3-1/2″ | — | 2-1/4″ | — | — | M56(56 mm) |
| 3-3/4″ | 2-1/2″ | — | — | — | M64(64 mm) |
| 3-7/8″ | — | 2-1/2″ | — | — | M64(64 mm) |
| 4″No ASME B18.2.1 hex bolt in this range | — | — | — | — | — |
Indicative reference only — does not replace a qualified technician. For safety-critical fasteners (wheels, steering, suspension, structural), measure the shank with a caliper and confirm thread pitch; do not rely on wrench size alone.
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