Quick Answer: Are metric and imperial Allen keys the same?
No, metric and imperial (SAE) Allen keys are not the same. While some sizes are very close and may seem to fit, using an imperial key on a metric bolt (or vice versa) leaves tiny gaps. Applying torque will cause the key to slip, ultimately stripping the bolt and damaging your tool.
Why You Need Both Metric and Imperial Hex Keys
Whether you are building flat-pack furniture, adjusting a bicycle, or working on a car engine, you will inevitably encounter both metric (millimetres) and imperial (fractions of an inch) fasteners.
It is crucial to use the exact size. For example, a 4mm hex key is 0.157 inches, whilst a 5/32" hex key is 0.156 inches. They are incredibly close, but applying heavy pressure with the wrong size can ruin the fastener.
Common Metric to Imperial Hex Key Conversions
While you should always use the correct dedicated set, here is a helpful reference chart showing standard sizes and their closest counterparts:
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1.5mm = Approx. 1/16"
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2.0mm = Approx. 5/64"
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2.5mm = Approx. 3/32"
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3.0mm = Approx. 7/64" (Loose fit)
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4.0mm = Approx. 5/32" (Very close)
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5.0mm = Approx. 3/16"
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6.0mm = Approx. 1/4" (Loose fit)
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8.0mm = Approx. 5/16"
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10.0mm = Approx. 3/8"
Stop Guessing Your Sizes
Constantly guessing whether you need a metric or imperial key wastes time and ruins hardware. The best solution is to keep a comprehensive, dual-measurement folding set or a colour-coded L-key set in your toolbox so the perfect fit is always within reach. View the HemBorta range to find your perfect key set today.
