What size spanner or socket does an M14 bolt take?

Answer 21 mm

An M14 hex bolt takes 21 mm across the flats — spanner or socket, and the same for its nut. A socket-head cap screw of the same size takes 12 mm. Older DIN 931/933 bolts of this size take 22 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 headM14 → 21 mm (old DIN: 22 mm)
Hex (Allen) key — socket-head cap screwM14 → 12 mm
The nut takes the same spanner21 mm fits the M14 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): 21 mm, or 22 mm on older DIN 931/933 bolts
Socket size: The same as the spanner — 21 mm.
Hex (Allen) key: 12 mm for a socket-head cap screw of this thread.
The nut: A standard M14 hex nut takes the same 21 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

M1218 mm M1624 mm M1016 mm M1827 mm M813 mm M2030 mm M610 mm M2234 mm M58 mm M47 mm

The rest of the job at this size

Tap drill for M14 × 212 mm Tightening torque for M14 × 2every grade

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