What size spanner or socket does an M12 bolt take?

Answer 18 mm

An M12 hex bolt takes 18 mm across the flats — spanner or socket, and the same for its nut. A socket-head cap screw of the same size takes 10 mm. Older DIN 931/933 bolts of this size take 19 mm instead — worth checking on anything not recently made.

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How it is worked out

Spanner / socket — across the flats of the headM12 → 18 mm (old DIN: 19 mm)
Hex (Allen) key — socket-head cap screwM12 → 10 mm
The nut takes the same spanner18 mm fits the M12 hex nut too

A standard hex is about 1.5x the thread diameter across the flats; the exact figure is fixed by the standard, not the formula

Worth knowing

Spanner size (across flats): 18 mm, or 19 mm on older DIN 931/933 bolts
Socket size: The same as the spanner — 18 mm.
Hex (Allen) key: 10 mm for a socket-head cap screw of this thread.
The nut: A standard M12 hex nut takes the same 18 mm.
Why two metric answers exist: ISO 4014 reduced the head across three sizes when it replaced the older DIN heavy series, so an M10 is now 16 mm where it used to be 17. Both are in service; measure if it matters.
This is the head, not the thread: The spanner fits the head. What the bolt threads into is the tap drill, and how hard to do it up is the torque — both linked below.

Spanner and socket size — nearby sizes

M1016 mm M1421 mm M813 mm M1624 mm M610 mm M1827 mm M58 mm M47 mm M2030 mm M35.5 mm

The rest of the job at this size

Tap drill for M12 × 1.7510.2 mm Tightening torque for M12 × 1.75every grade

Do it with your own numbers

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