Hex Key Size Chart: SAE to Metric

Across the range where both standards exist, only six metric hex keys have an imperial equivalent inside a usable tolerance, and just three of those are at sizes you would meet on a normal job: 3.5 mm with 9/64", 4 mm with 5/32" and 8 mm with 5/16". Everything else is close enough to feel right and far enough to do damage. The chart below gives the exact gap for every size and, more importantly, which way it errs.

Why a hex key is less forgiving than a spanner

A spanner grips two flats from the outside and can be nudged, re-seated or swapped for a socket. A hex key has to enter a closed recess and bear on all six flats from within, with nothing outside the fastener to hold on to. That single difference halves the usable tolerance: about five thousandths of an inch is workable on a spanner, and about three is the limit on a hex key.

It also makes the direction of the error decisive, which a plain difference column hides. An imperial key larger than the metric socket will not go in — irritating, obvious, harmless. One that is smaller goes in, turns, feels perfectly normal, and rounds the recess out from the inside. By the time it slips there is nothing left for the correct key to grip either, and the screw has to come out by drilling.

Metric to SAE hex key conversion

Method and precision

Decimal inch values are exact at 25.4 mm to the inch. The difference column is the imperial size minus the metric size in thousandths of an inch, so a negative figure means the imperial key is the smaller of the two and will enter a socket it does not fill. The verdict is set at three thousandths, tighter than the five used for spanners because a hex key bears inside a closed recess with no purchase anywhere else. Metric sizes are the set defined by ISO 2936; imperial sizes are the ANSI/ASME B18.3 list, which stops at 3/8 inch, so the cross-reference runs only as far as the two systems genuinely overlap.

The pairings that actually work

Only six metric sizes have an imperial key inside tolerance, and three of them are the very small ones: 0.7 mm, 0.9 mm and 1.3 mm are the decimal-inch keys 0.028 inch, 0.035 inch and 0.050 inch under another name, and were designed to be. Above those the complete list is 3.5 mm with 9/64", 4 mm with 5/32" and 8 mm with 5/16". Nothing else in either standard comes close enough to use.

The ones to be careful with are the four that resolve to an undersize imperial key: 2.5 mm, 5 mm, 7 mm and 10 mm. Each of those hands you a key that drops straight in and gives no warning at all until it slips. 10 mm against 3/8 inch is the worst of them at nearly nineteen thousandths, and it is also one of the most commonly reached for.

Every metric hex key size, and what it drives

The metric roster runs well past the point where an imperial key exists at all. Below is the full defined set with the socket head cap screw thread each size drives, so a key can be picked from the fastener rather than by trial.

Full ranges are in hex and TX keys, bits and drivers and sockets and accessories. The screw dimensions behind the last column are in the socket head cap screw chart.