How to Measure a Hex Key | Size by Across-Flats

For the full breakdown, see our best Tool Hex Key guide.

Grab a misplaced Allen wrench and you face a small puzzle: which of the ten sizes in that drawer is this one? The answer lives in one dimension. A hex key is measured across its flats, meaning the distance between two opposite parallel flat sides of the hexagonal tip. That single measurement, taken where the key does its work, tells you exactly which screw it fits.

What Exactly Are You Measuring?

Hex keys get their name from their hexagonal cross-section, and that shape defines how you measure them. The size is the width across flats — the straight-line distance spanning two opposite flat surfaces of the hexagon. This is the dimension that matches the socket in a screw head.

Three traps send people astray. Do not measure the overall length of the key, Do not measure the shank. Do not measure corner-to-corner across the hexagon’s points. Any of those will give you a number that looks plausible but won’t match a size chart. If you want to identify a key you found loose in a toolbox, you have to find its flat-to-flat width.

Using Calipers for An Accurate Reading

A digital or dial caliper is the right instrument for this job. It reads directly in millimeters or inches, and its jaws grip the flats squarely enough to give you a trustworthy number.

1. Close the caliper jaws fully and zero the display.
2. Open the jaws and place them squarely on two opposite flat sides of the hex key’s working end.
3. Close the jaws snugly against the flats — firm, but do not crush the metal.
4. Read the measurement. That number is your hex key size.

If the key shows wear at the tip, take readings at a couple of points along the usable length and use the average. A worn corner can read small and send you hunting for the wrong socket.

No calipers on hand? Lay the key across a ruler and eyeball the flat-to-flat width, or press it into a dedicated hex key gauge. These methods work, but they sacrifice accuracy — fine for a quick check, not for precision work where a snug fit matters.

Matching Your Measurement to A Size Chart

Once you have the across-flats number, match it to standard metric or SAE sizes. Most keys fall into these common equivalents:

If you are working to a spec, that standard is the authority. If you just need to turn a bolt, the table above covers most common sizes.

One important caveat: the hex key size is not the same as the screw’s nominal diameter. The across-flats dimension tells you the tool size; the fastener’s thread diameter and pitch are separate details you still need to verify when identifying a screw.

Why Getting This Right Matters

Choosing a size by eye is the fastest way to round off a screw head. When the key is too small, it slips and chews the socket. Too large, and it will not seat at all. Forcing either scenario can damage the screw or the key itself, turning a five-second job into a extraction project.

A snug fit is the goal. The key should seat fully with little visible play. If you feel slop, step up a size. If it binds before seating, step down. A properly fitted hex key transfers torque cleanly and makes stubborn fasteners come loose with less effort.

FAQs

Can I use a ruler instead of calipers?

Yes, a ruler works for a rough measurement, but calipers are far more accurate. Position the key flat against the ruler and read the width across its flats. This gets you close enough to identify most keys, though worn or beveled edges can skew the reading by a fraction of a millimeter.

Why does my hex key not match the screw size?

The hex key across-flats measurement rarely equals the screw’s nominal diameter. A 6 mm key, for instance, fits a screw with a larger thread diameter. The key size refers strictly to the hexagonal socket dimension, so always verify thread diameter separately when sizing a fastener.

What if my measurement falls between standard sizes?

A reading between standard sizes usually means the key is worn, or you caught a slightly beveled edge in the caliper jaws. Measure again at a different point along the usable length. If it still lands in between, treat it as the smaller size but check the fit carefully before applying torque.