Metric Bolt Head and Socket Size Chart

One chart for the two questions asked most at the bench: which socket fits this hex head, and which hex key fits this cap screw.

Metric hex-head bolts under ISO 4017 carry a head that is larger than the thread. The across-flats measurement — the distance between two opposite flats — is the socket size: M6 takes 10 mm, M8 takes 13 mm, M10 takes 17 mm, M12 takes 19 mm and M16 takes 24 mm.

Socket cap screws under ISO 4762 invert the logic: the tool goes inside the head, and the hex key runs one size below the thread. M6 takes a 5 mm key, M8 a 6 mm key, M10 an 8 mm key and M12 a 10 mm key.

A standard metric folding hex-key set holds nine keys — 1.5, 2, 2.5, 3, 4, 5, 6, 8 and 10 mm — which covers every cap screw in the chart with room to spare. The 6 mm key that an M8 cap screw wants is the fifth key in that set.

For sockets, a typical 3/8 in-drive set spans 8–19 mm and therefore covers the M6-to-M12 hex heads in one case. The M16 head at 24 mm usually means stepping up to a 1/2 in-drive socket.

Cross-heads follow their own scale: Phillips #00, #0, #1, #2 and #3, with #2 the standard for cabinet and deck screws and #1 for small electronics. A Phillips number describes the driver point, not the screw thread.

When a metric fastener meets an inch toolbox, check the near-miss pairs on the metric-versus-imperial page before forcing anything — 5/16 in against 8 mm is the pair that fools people most often.

• M6 hex head → 10 mm socket; M6 cap screw → 5 mm key
• M8 hex head → 13 mm socket; M8 cap screw → 6 mm key
• M10 hex head → 17 mm socket; M10 cap screw → 8 mm key
• M12 hex head → 19 mm socket; M12 cap screw → 10 mm key
• M16 hex head → 24 mm socket

Further reading

• Metric vs Imperial Fastener Sizes
• How to Remove a Stripped Bolt