What size spanner or socket does an M18 bolt take?

Answer 27 mm

An M18 hex bolt takes 27 mm across the flats — spanner or socket, and the same for its nut. A socket-head cap screw of the same size takes 14 mm.

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How it is worked out

Spanner / socket — across the flats of the headM18 → 27 mm
Hex (Allen) key — socket-head cap screwM18 → 14 mm
The nut takes the same spanner27 mm fits the M18 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): 27 mm
Socket size: The same as the spanner — 27 mm.
Hex (Allen) key: 14 mm for a socket-head cap screw of this thread.
The nut: A standard M18 hex nut takes the same 27 mm.
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

M1624 mm M2030 mm M1421 mm M2234 mm M1218 mm M2436 mm M1016 mm M2741 mm M813 mm M610 mm

The rest of the job at this size

Tap drill for M18 × 2.515.5 mm Tightening torque for M18 × 2.5every grade

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