Metric Bolt Head & Drive Sizes (ISO 4014)
Head and drive dimensions for metric hex bolts (ISO 4014 / 4017) and socket head cap screws (ISO 4762). Width across flats is the wrench or socket size you need.
| Size | Coarse pitch (mm) | Across flats s (mm) | Across corners e (mm) | Head height k (mm) | Hex key (mm) |
|---|
Standard: ISO 4014 / ISO 4017 / ISO 4762 · Data checked: 2026-10-14
Frequently asked questions
Why does a 17 mm socket not fit an M10 bolt?
Because there are two metric head standards and they disagree. The withdrawn DIN 931 and 933 used 17, 19 and 22 mm across flats at M10, M12 and M14. ISO 4014, which replaced them, uses 16, 18 and 21 mm. A socket set bought for old stock will not fit new bolts, and the reverse is also true.
What is the difference between a bolt and a screw?
In practice, a bolt is tightened with a nut and a screw threads into the part itself, but the standards do not use the words that consistently. What matters for dimensions is the head type — hex head under ISO 4014 or 4017, socket head under ISO 4762 — not what it is called.
Why does the head height matter?
Because it sets how deep a counterbore has to be and how much clearance the head needs around it. A socket head cap screw has a head height roughly equal to its diameter, where a hex head is only about two-thirds of that, so they are not freely interchangeable in a tight recess.
What does across corners mean and why is it bigger?
It is the distance across opposite corners of the hexagon, and it is always larger than the across-flats dimension. It matters because a twelve-point socket, or a worn six-point one, has to clear it — a socket that only just fits across flats may not go on at all.