Which wrench or socket fits a bolt, and which hex key fits a cap, button or flat head screw. SAE and metric.
| Bolt | Wrench / socket | Hex key | |||
|---|---|---|---|---|---|
| Hex bolt | Hex nut | Socket cap | Button head | Flat head | |
| #4 | — | 1/4″ | 3/32″ | 1/16″ | 1/16″ |
| #6 | — | 5/16″ | 7/64″ | 5/64″ | 5/64″ |
| #8 | — | 11/32″ | 9/64″ | 3/32″ | 3/32″ |
| #10 | — | 3/8″ | 5/32″ | 1/8″ | 1/8″ |
| 1/4″ | 7/16″ | 7/16″ | 3/16″ | 5/32″ | 5/32″ |
| 5/16″ | 1/2″ | 1/2″ | 1/4″ | 3/16″ | 3/16″ |
| 3/8″ | 9/16″ | 9/16″ | 5/16″ | 7/32″ | 7/32″ |
| 7/16″ | 5/8″ | 11/16″ | 3/8″ | — | 1/4″ |
| 1/2″ | 3/4″ | 3/4″ | 3/8″ | 5/16″ | 5/16″ |
| 9/16″ | 13/16″ | 7/8″ | — | — | — |
| 5/8″ | 15/16″ | 15/16″ | 1/2″ | 3/8″ | 3/8″ |
| 3/4″ | 1-1/8″ | 1-1/8″ | 5/8″ | — | 1/2″ |
| 7/8″ | 1-5/16″ | 1-5/16″ | 3/4″ | — | — |
| 1″ | 1-1/2″ | 1-1/2″ | 3/4″ | — | — |
| Bolt | Hex head wrench | Hex key | |||
|---|---|---|---|---|---|
| ISO 4017 | DIN 933 | Socket cap | Button head | Flat head | |
| M3 | 5.5 | 5.5 | 2.5 | 2 | 2 |
| M4 | 7 | 7 | 3 | 2.5 | 2.5 |
| M5 | 8 | 8 | 4 | 3 | 3 |
| M6 | 10 | 10 | 5 | 4 | 4 |
| M8 | 13 | 13 | 6 | 5 | 5 |
| M10 | 16 | 17 | 8 | 6 | 6 |
| M12 | 18 | 19 | 10 | 8 | 8 |
| M14 | 21 | 22 | 12 | — | — |
| M16 | 24 | 24 | 14 | 10 | 10 |
| M20 | 30 | 30 | 17 | — | — |
| M24 | 36 | 36 | 19 | — | — |
M10, M12 and M14 head sizes changed when ISO 4017 replaced DIN 933. Older and imported hardware often uses the DIN size, so check both when a wrench doesn't fit. A dash means no standard size.
A bolt is named by its thread diameter, not its head. A 3/8″ bolt has a 3/8″ thread and a 9/16″ head, so it takes a 9/16″ wrench. Metric works the same way: an M8 bolt has an 8 mm thread and a 13 mm head.
Hex keys go by the size of the socket in the head, which depends on the head style: socket head cap screws take a bigger key than button and flat head screws of the same thread.