What size spanner or socket does an M8 bolt take?

Answer 13 mm

An M8 hex bolt takes 13 mm across the flats — spanner or socket, and the same for its nut. A socket-head cap screw of the same size takes 6 mm.

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How it is worked out

Spanner / socket — across the flats of the headM8 → 13 mm
Hex (Allen) key — socket-head cap screwM8 → 6 mm
The nut takes the same spanner13 mm fits the M8 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): 13 mm
Socket size: The same as the spanner — 13 mm.
Hex (Allen) key: 6 mm for a socket-head cap screw of this thread.
The nut: A standard M8 hex nut takes the same 13 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

M610 mm M1016 mm M58 mm M47 mm M1218 mm M35.5 mm M1421 mm M1624 mm M1827 mm M2030 mm

The rest of the job at this size

Tap drill for M8 × 1.256.8 mm Tightening torque for M8 × 1.25every grade

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