Hex screwdrivers - also called Allen drivers - fit hexagonal socket head cap screws. The six-sided recess provides excellent torque transfer with no cam-out. Hex fasteners are everywhere: furniture, bicycles, motorcycles, machinery, and socket head cap screws in engineering applications.
Metric vs Imperial Hex Sizes
Metric and imperial hex sizes are close but not interchangeable. Forcing a 5/32″ key into a 4 mm socket (or vice versa) rounds out the hex recess. Always check whether your fasteners are metric or imperial.
| Metric (mm) | Imperial (inch) | Difference | Common Application |
|---|---|---|---|
| 1.5 mm | 1/16″ (1.59 mm) | 0.09 mm | Set screws, small electronics |
| 2.0 mm | 5/64″ (1.98 mm) | 0.02 mm | Grub screws, small fixtures |
| 2.5 mm | 3/32″ (2.38 mm) | 0.12 mm | Furniture, bike accessories |
| 3.0 mm | 1/8″ (3.18 mm) | 0.18 mm | Furniture assembly (IKEA), bike parts |
| 4.0 mm | 5/32″ (3.97 mm) | 0.03 mm | Bike components, machinery |
| 5.0 mm | 3/16″ (4.76 mm) | 0.24 mm | Bike stems, furniture, general engineering |
| 6.0 mm | 1/4″ (6.35 mm) | 0.35 mm | Socket cap screws, structural |
| 8.0 mm | 5/16″ (7.94 mm) | 0.06 mm | Large socket cap screws, automotive |
| 10.0 mm | 3/8″ (9.53 mm) | 0.47 mm | Heavy structural, industrial |
Hex Driver Formats
- L-key (Allen wrench) - the classic bent hexagonal rod. Cheap, compact, and included with most flat-pack furniture
- T-handle - an L-key with a handle for more torque and comfort during extended use
- Screwdriver-style - hex bit in a full screwdriver handle. Best for workshop use
- Bit set - hex bits that fit into a bit driver, ratchet, or power tool
- Ball-end - a rounded tip on the long arm that allows driving at angles up to 25°
Pro Tip
Ball-end hex keys let you access fasteners at an angle. The long arm with the ball tip goes into the screw for angled access; the short arm provides full torque for final tightening.
