Hex Key Size Gauge — Metric and SAE Allen Keys

A free tool for sizing an Allen key when metric and inch keys are jumbled in one set. A hex key is named by its across-flats width — the gap between two opposite faces — and the two standards sit dangerously close: a 5 mm key is 0.24 mm fatter than a 3/16" one, and 6 mm barely undercuts 1/4". Pick the wrong twin and you round the socket. Slot the short arm into the holes, or lay it flat on a printed scale, and read the across-flats size in both systems.

Identifying a hex key step by step

1
Use the straight arm

Measure the plain hex part of the short arm, away from any ball end, where all six faces are full width.

2
Lay a flat down

Rest one flat face on the scale so you read across two parallel faces — across the flats — not corner to corner.

3
Or drop it in a hole

Try the sizing holes; the smallest one the arm fills without play is its across-flats size.

4
Confirm the standard

The tool gives the metric and SAE reading and flags when the two are close enough that only the snug one will fit safely.

Tell a near-identical metric key from its SAE twin

What this gauge does

Across-flats, measured flat
Reads the flat-to-flat width that actually drives the bolt, taken with a face resting on the scale rather than across a corner.
Spots the deceptive pairs
Highlights the close calls that strip sockets — 5 mm beside 3/16", 6 mm beside 1/4", 8 mm beside 5/16" — so you choose the snug fit, not the loose one.
Metric and SAE ranges
Covers metric keys from 0.7 to 12 mm and SAE keys from 0.028" to 3/8", the spans found in furniture, bike and machine sets.
Hole-fit check
The arm drops into the smallest hole it fills, confirming the size by fit — the same test the bolt socket applies.
Ball-end aware
Reminds you to gauge on the straight hex section, never the rounded ball end, which measures undersize and lies about the key.

Why eyeballing a hex key strips bolts

Hex sets mix metric and inch keys that differ by a fraction of a millimetre, and the cost of guessing is a rounded socket you can no longer turn. A ruler cannot resolve 5 mm from 3/16" by eye, but a snug hole-fit can. This tool reads the across-flats width the way the bolt does, names it in both standards and warns when the metric and SAE sizes are close enough that only the right one will hold — so the key bites the flats instead of chewing the corners.

Hex key sizing questions

What does "across the flats" mean?

It is the distance between two opposite flat faces of the hexagon — the width that matches the bolt's socket. It is smaller than the corner-to-corner distance, so always measure flat-to-flat, with one face resting on the scale.

Which metric and inch sizes are easiest to confuse?

The dangerous pairs are 5 mm versus 3/16" (4.76 mm), 6 mm versus 1/4" (6.35 mm) and 8 mm versus 5/16" (7.94 mm). Each pair is within about a quarter of a millimetre, so the wrong one slips and rounds the fastener. The tool calls these out by name.

Why does the wrong-size key strip a bolt?

A key that is slightly too small contacts only the corners of the socket instead of the faces. Under load those corners deform and the socket rounds out. Choosing the snug fit of the correct standard spreads the force across the flats and prevents it.

Can I just use the closest size I own?

Only if it is the same standard and a tight fit. A metric bolt wants a metric key; reaching for the nearest SAE key because it "almost fits" is the classic way to ruin a socket. Identify the standard first, then use that key.

Where do I measure on a ball-end key?

On the straight hexagonal shaft, not the ball. The ball end is ground smaller so it can pivot in the socket, so measuring there would undersize the key. The full across-flats width is on the plain section.

What about Torx, square or spline keys?

This gauge is for six-sided hex (Allen) keys, sized across the flats. Torx star, square (Robertson) and spline drives use their own size codes and a different cross-section, so they are not read on this scale.

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