Allen Wrench Conversion Standard Imperial Metric

Allen Wrench Conversions

Allen Wrench Conversions SAE Metric Hex Key

The Ultimate Guide to Allen Wrench Conversions

Sometimes—you just have to improvise when you’re on the job. So we decided to make that a little bit easier for you. If you ever need to lookup an Allen wrench conversion, keep our guide handy so you can reference it when out in the field. We’ve provided the best substitute for popular hex sizes from both Imperial & Metric measurement systems. But to ensure that you don’t damage your tools or your hardware, take a look at our instructions below for tips & recommendations.

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.

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.

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 Substitute for SI

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


SI Substitute 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


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