SAE bolt size to wrench size
Standard hex heads follow one rule: across flats is one and a half times the thread diameter for 1/4 through 5/8, then rounded up in sixteenths above that. Heavy hex heads add 1/8 inch and are used on structural bolts.
| Thread | Standard hex | Heavy hex | Metric spanner that fits |
|---|---|---|---|
| 1/4 | 7/16 | 1/2 | 11 mm |
| 5/16 | 1/2 | 9/16 | 13 mm |
| 3/8 | 9/16 | 11/16 | 14 mm |
| 7/16 | 5/8 | 3/4 | 16 mm |
| 1/2 | 3/4 | 7/8 | 19 mm |
| 9/16 | 13/16 | 15/16 | 21 mm |
| 5/8 | 15/16 | 1 1/16 | 24 mm |
| 3/4 | 1 1/8 | 1 1/4 | 29 mm |
| 7/8 | 1 5/16 | 1 7/16 | 34 mm |
| 1 | 1 1/2 | 1 5/8 | 38 mm |
Metric bolt size to spanner size
Two standards are in circulation and they disagree at three sizes. ISO 4014 gives M10 a 16mm head, M12 an 18mm head and M14 a 21mm head. The older DIN 931 gives those same bolts 17, 19 and 22mm. Both are still sold, so a set needs both sizes.
| Thread | ISO 4014 | DIN 931 | Nut (DIN 934) |
|---|---|---|---|
| M4 | 7 | 7 | 7 |
| M5 | 8 | 8 | 8 |
| M6 | 10 | 10 | 10 |
| M8 | 13 | 13 | 13 |
| M10 | 16 | 17 | 17 |
| M12 | 18 | 19 | 19 |
| M14 | 21 | 22 | 22 |
| M16 | 24 | 24 | 24 |
| M20 | 30 | 30 | 30 |
| M24 | 36 | 36 | 36 |
SAE spanner sizes in millimetres
When you only have a metric set, this is what actually fits. Anything closer than about 0.1mm works cleanly; wider than that and the corners round under load.
| SAE | mm | Nearest metric | Fit |
|---|---|---|---|
| 7/16 | 11.11 | 11 | Good |
| 1/2 | 12.70 | 13 | Loose |
| 9/16 | 14.29 | 14 | Loose |
| 5/8 | 15.88 | 16 | Good |
| 11/16 | 17.46 | 17 | Poor |
| 3/4 | 19.05 | 19 | Good |
| 13/16 | 20.64 | 21 | Loose |
| 7/8 | 22.23 | 22 | Loose |
| 15/16 | 23.81 | 24 | Loose |
| 1 | 25.40 | 25 | Poor |
