Allen Wrench Conversions
The Ultimate Guide to Allen Wrench Conversions
Imperial to Metric for Allen Keys
On the left-hand side, we have our SAE to metric conversion guide for hex keys that provides the best substitute SAE Allen wrench for each given metric size. As you’ll notice, there isn’t always a perfect substitute. For instance, Metric size 9 mm doesn’t have an imperial counterpart. In a case like this, you’re only option is to just upgrade to a separate metric hex key set. With more and more hardware being produced internationally, both SAE and metric hex key sets will have a place in your tool set.
Metric to Imperial for Allen Keys
Moving over to the right-hand side, we’ve provided the metric to SAE guide for Allen wrench conversions. Similar to the metric hex keys, the SAE hex wrenches don’t always have a perfect substitute as seen for SAE size 3/16″. Thus, we recommend that those with only a metric Allen wrench set add a separate SAE hex key set to their stable.
Substituting Uncommon Hex Sizes
Lastly, there’s one other added benefit to having both SAE & metric hex keys. In the United States, larger metric half sizes like 6.5 mm aren’t standard in most hex key sets. However, the 1/4″ SAE hex wrench will likely work in place of a 6.5 mm wrench. In scenarios like these, use the chart to estimate the correct alternative. You can test different sizes till you find the best match. We do recommend starting slowly to ensure that you don’t damage the hardware.
Pro Tips:
- 3/8″ can be used as a substitute for hex size 9.5 mm as well as 10 mm
- 1/16″ works in place of hex size 1.5 mm, which isn’t included in most sets
When you reference the hex key conversion chart above, do be aware that using a smaller size bit will increase your vulnerability for stripping the hardware. In some cases, you may want to try using a size larger than the one recommended in the chart to see if it fits. Also, we will caution you that smaller screw sizes are more prone to stripping. Thus, be careful when substituting sizes with precision hardware. Lastly, do remember that not every Allen wrench size has a perfect counterpart. That’s it when it comes to comparing metric & standard Allen wrench sizes. Good luck!
SAE Substitutes for MM
| Imperial Hex Key | Metric Hex Size |
| 5/64″ | 2 mm |
| 3/32″ | 2.5 mm |
| 7/64″ | 3 mm |
| 1/8″ | 3.5 mm |
| 5/32″ | 4 mm |
| 3/16″ | 5 mm |
| 7/32″ | 5.5 mm & 6 mm |
| 1/4″ | 7 mm |
| 5/16 “ | 8 mm |
| 3/8″ | 10 mm |
MM Substitutes for SAE
| Metric Hex Key | Imperial Hex Size |
| 2 mm | 3/32 |
| 2.5 mm | 7/64 |
| 3 mm | 1/8 |
| 3.5 mm | 9/64 |
| 4 mm | 5/32 |
| 5.5 mm | 7/32 |
| 6 mm | 1/4 |
| 7 mm | 9/32 |
| 8 mm | 5/16 |
| 9 mm | 3/8 |
Also published on Medium.
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