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About this tool
Matches metric and imperial hex key sizes, shows which swaps are safe and which round the socket, and prints a true-scale gauge.
Matches hex key sizes across the metric and imperial systems and grades every swap by clearance, the number that decides whether a substitution works or destroys the fastener. A key that leaves under 1% slack in the socket is the same tool with a different name; 1 to 3% turns a lightly torqued screw; over 3% loads the corners of the socket instead of the flats and rounds it out. Fastener sizes come from the published standards — ISO 4762, 7380, 10642 and 4026, and ASME B18.3 for the imperial series.
Open Allen Key Size Finder on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Every pair is rated as interchangeable, usable with care, or a socket-rounder.
A key safe in a metric socket is usually too wide for the nearest imperial one, and the tool says so.
Cap, button, countersunk and grub screws of the same thread take different keys.
Effectively yes. 5/16 inch is 7.9375 mm, so it sits 0.0625 mm inside an 8 mm socket — under 1% clearance, which is within the tolerance the two parts are made to anyway. It is one of only three pairs that genuinely interchange, the others being 5/32 inch with 4 mm and 5/64 inch with 2 mm.
No, in that direction it will not even enter: 1/4 inch is 6.35 mm and the socket is 6. Going the other way is worse — a 6 mm key in a 1/4 inch socket leaves 5.5% clearance, which puts the load on the corners of the socket and rounds it. This is the single commonest way people wreck a socket screw.
It depends on the head. An M6 socket head cap screw to ISO 4762 takes a 5 mm key, an M6 button head to ISO 7380 takes 4 mm, and an M6 socket set screw to ISO 4026 takes 3 mm. Head style changes the socket, so the thread size alone does not tell you the key.
Measure across the flats, not corner to corner, with a vernier caliper — that width is the size. Failing that, print a true-scale gauge and lay the short leg across each bar, or try keys in a known socket screw. Guessing by eye between 5 mm and 3/16 inch is exactly the mistake that rounds sockets.