Hex Key Size Chart: Metric to Imperial
Overview
Size reference for metric and imperial hexagonal keys (Allen wrenches). Use the “Difference” column to determine if a cross-size is safe to use.
Comparison Table
| Metric Hex (mm) | Nearest Imperial | Imperial (mm) | Difference | Best Fit |
|---|---|---|---|---|
| 2.0 mm | 5/64” | 1.98 mm | 0.02 mm | Interchangeable |
| 2.5 mm | 3/32” | 2.38 mm | 0.12 mm | Metric Only |
| 4.0 mm | 5/32” | 3.97 mm | 0.03 mm | Interchangeable |
| 5.0 mm | 3/16” | 4.76 mm | 0.24 mm | Metric Only |
| 6.0 mm | 7/32” | 5.56 mm | 0.44 mm | Metric Only |
| 8.0 mm | 5/16” | 7.94 mm | 0.06 mm | Interchangeable |
FAQ
Can I use a metric hex key on an imperial bolt?
Only if the difference is less than 0.05 mm (indicated as “Interchangeable” above). Using a key with a difference larger than 0.1mm will likely strip the internal socket of the bolt.
Source Standard
ISO 2936 / ASME B18.3