The last column is the one you came for: M6 takes a 5 mm key, M8 takes 6 mm, M10 takes 8 mm and M12 takes 10 mm. The rest of the table is head diameter and head height, which is what decides whether a head clears its counterbore. ISO 4762 and DIN 912 are the same screw — the current designation is DIN EN ISO 4762 — so a drawing calling for either gives the dimensions below.
Reading the table
Head diameter and head height are maximum figures, which is the useful direction: a counterbore sized on them clears the largest head the standard permits, and anything within tolerance will drop into it. The hex socket size is nominal across the flats, and it is the dimension the key has to match — not the thread, and not the head.
The relationship between thread and key is not a formula, which is why a chart is needed at all. It does not step evenly: M6 to M8 goes from a 5 mm to a 6 mm key, but M12 to M16 skips from 10 mm straight to 14 mm with nothing in between, because M14 is not part of the preferred thread series.
Socket head cap screw dimensions
| Thread | Pitch (mm) | Head diameter (mm) | Head height (mm) | Hex key (mm) |
|---|---|---|---|---|
| M1.6 | 0.35 | 3 | 1.6 | 1.5 |
| M2 | 0.4 | 3.8 | 2 | 1.5 |
| M2.5 | 0.45 | 4.5 | 2.5 | 2 |
| M3 | 0.5 | 5.5 | 3 | 2.5 |
| M4 | 0.7 | 7 | 4 | 3 |
| M5 | 0.8 | 8.5 | 5 | 4 |
| M6 | 1 | 10 | 6 | 5 |
| M8 | 1.25 | 13 | 8 | 6 |
| M10 | 1.5 | 16 | 10 | 8 |
| M12 | 1.75 | 18 | 12 | 10 |
| M16 | 2 | 24 | 16 | 14 |
| M20 | 2.5 | 30 | 20 | 17 |
| M24 | 3 | 36 | 24 | 19 |
| M30 | 3.5 | 45 | 30 | 22 |
| M36 | 4 | 54 | 36 | 27 |
Method and precision
Dimensions are ISO 4762, which DIN 912 has been superseded by and is dimensionally identical to. Head diameter and head height are maximum values; the hex socket is the nominal size across the flats. Pitch is the coarse-series pitch for each thread. M14 is deliberately absent: it is not in the preferred series and is not in the source table, and a row invented to close the gap between M12 and M16 would look exactly as authoritative as the real ones. Every key size listed is cross-checked against the sizes ISO 2936 actually defines, so no screw here calls for a key that is not made.
Two patterns in the table are worth noticing because they are properties of the standard rather than coincidences. Head height equals the nominal thread diameter at every single size — an M10 head is 10 mm tall, an M24 head is 24 mm tall. And from M12 upwards, head diameter is exactly one and a half times the thread diameter.
Counterbore and clearance
A socket head is designed to sit inside the work, so the hole it drops into has to clear the head diameter above with a little to spare, and be deep enough for the head height plus whatever recess the drawing wants. The figures above are maxima for exactly this reason: size a counterbore on them and every screw within tolerance goes in, size it on a nominal figure and the occasional one does not.
Depth matters as much as diameter. A counterbore cut to exactly the head height leaves the head flush, which is fine until a washer appears, and a head that sits proud of a mating face is a joint that will not close.
What actually rounds these
Socket heads round from the inside, and almost always because the key was slightly undersize — an imperial key in a metric socket, or a worn key whose corners have gone. The recess gives no warning: it takes load normally right up to the point the key spins, and by then there is nothing left for the correct key to grip. A ball-end key makes it more likely again, because it bears on a smaller area by design and is meant for reaching an angle, not for final tightening.
The imperial-against-metric question is worked through in full in the hex key size chart. Keys are in hex and TX keys, socket bits in bits and drivers, and the drivers for them in sockets and accessories.