Quick Answer: How do you measure an Allen key size?
To measure an Allen (hex) key, use a set of digital calipers to measure the distance flat-to-flat (across the width of the hexagon), not corner-to-corner. For a Torx key, measure the distance point-to-point across the widest part of the star shape.
Stop Guessing Your Tool Sizes
We've all been there: you have a handful of loose keys in a drawer, the painted size markings have rubbed off, and you have no idea which one you are holding. Knowing exactly how to measure your fasteners and tools saves time and prevents stripped bolts.
How to Measure an Allen (Hex) Key
Because Allen keys are hexagonal, measuring them is straightforward.
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Take a set of Vernier calipers (digital ones are best for accuracy).
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Place the jaws of the caliper flat against two opposite, parallel flat sides of the hex key.
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Do not measure from corner to corner. This will give you an incorrect, oversized reading.
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Read the measurement. If it reads 5.0mm, you have a 5mm metric key. If it reads roughly 6.35mm, you likely have a 1/4" imperial key.
How to Measure Torx Sizes
Torx sizes are denoted by a "T" number (e.g., T10, T25, T30). These numbers correspond to specific point-to-point measurements.
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Use your calipers to measure across the widest part of the star—from the very tip of one point to the tip of the point directly opposite.
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Cross-reference your measurement with a standard Torx chart. For example, a point-to-point measurement of 2.74mm is a T10, whilst 4.43mm is a T25.
If you find yourself constantly measuring loose keys, it might be time to upgrade to a HemBorta tool set that features clear, high-visibility size markings stamped directly into the metal.
