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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 is nominally larger than the socket does not enter at all; among keys that do fit, one that leaves under 1% slack is the same tool with a different name, 1 to 3% turns a lightly torqued screw, and 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.
Pick 'The key or socket you have' — Metric, in millimetres or Imperial, in inch fractions — then the 'Size across flats' from that system's list; the hint below converts the size into the other unit.
Read the 'Closest in the other system' headline with the millimetre comparison, and the colour-coded verdict beneath it that says whether the swap is interchangeable, needs care, will round the socket or is too large to enter.
In 'Key size for a fastener' choose a Fastener standard — Socket head cap screw (ISO 4762 / DIN 912), Button head screw (ISO 7380), Countersunk socket screw (ISO 10642), Socket set screw / grub screw (ISO 4026) or ASME B18.3 — and a Screw size; Copy result copies the match, and a cross-reference table and printable gauge sit below.
Every pair is rated as interchangeable, usable with care, a socket-rounder, or simply too large to enter the socket at all.
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.
Only in one direction. 5/16 inch is 7.9375 mm, so a 5/16 inch key drops into an 8 mm socket with just 0.0625 mm of play — under 1% clearance, effectively the same size. Going the other way is a different question: an 8 mm key is nominally 0.0625 mm wider than a 5/16 inch (7.9375 mm) socket, so treat it as the tighter fit and reach for the inch key when you have the choice. The same one-way relationship holds for 5/32 inch with 4 mm and 5/64 inch with 2 mm — the inch key is the safe one to put in the metric socket, not the reverse.
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.