The wrench size is the width across the flats (A/F) of the hex — the distance between two opposite faces, which is what a spanner or socket grips. It is set by the fastener standard and is not the thread (nominal) size: an M10 bolt has a 10 mm thread but a 16 mm A/F. Current ISO 4014/4017 and DIN 933/934 give the metric values above, but older DIN editions used a few larger heads — M10 was 17 mm, M12 was 19 mm and M22 was 32 mm — so on legacy or mixed hardware measure the actual head before reaching for a spanner. Socket sizes follow the same A/F, so the metric or inch number that fits a bolt is identical whether you use a wrench or a socket.
| Bolt size | System | Across flats (mm) | Across flats (in) |
|---|---|---|---|
| M3 | Metric | 5.5 | 0.217 |
| M4 | Metric | 7 | 0.276 |
| M5 | Metric | 8 | 0.315 |
| M6 | Metric | 10 | 0.394 |
| M8 | Metric | 13 | 0.512 |
| M10 | Metric | 16 | 0.63 |
| M12 | Metric | 18 | 0.709 |
| M14 | Metric | 21 | 0.827 |
| M16 | Metric | 24 | 0.945 |
| M18 | Metric | 27 | 1.06 |
| M20 | Metric | 30 | 1.18 |
| M22 | Metric | 34 | 1.34 |
| M24 | Metric | 36 | 1.42 |
| 1/4 | Inch (SAE) | 11.1 | 7/16 |
| 5/16 | Inch (SAE) | 12.7 | 1/2 |
| 3/8 | Inch (SAE) | 14.3 | 9/16 |
| 7/16 | Inch (SAE) | 15.9 | 5/8 |
| 1/2 | Inch (SAE) | 19.1 | 3/4 |
| 9/16 | Inch (SAE) | 20.6 | 13/16 |
| 5/8 | Inch (SAE) | 23.8 | 15/16 |
| 3/4 | Inch (SAE) | 28.6 | 1-1/8 |
| 7/8 | Inch (SAE) | 33.3 | 1-5/16 |
| 1 | Inch (SAE) | 38.1 | 1-1/2 |
Source: Current ISO 4014/4017 & DIN 933/934 (metric), ASME B18.2.1 (inch). Older DIN parts used M10→17, M12→19, M22→32 — check the actual head.
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