What size spanner or socket does an M4 bolt take?

Answer 7 mm

An M4 hex bolt takes 7 mm across the flats — spanner or socket, and the same for its nut. A socket-head cap screw of the same size takes 3 mm.

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How it is worked out

Spanner / socket — across the flats of the headM4 → 7 mm
Hex (Allen) key — socket-head cap screwM4 → 3 mm
The nut takes the same spanner7 mm fits the M4 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): 7 mm
Socket size: The same as the spanner — 7 mm.
Hex (Allen) key: 3 mm for a socket-head cap screw of this thread.
The nut: A standard M4 hex nut takes the same 7 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

M35.5 mm M58 mm M610 mm M813 mm M1016 mm M1218 mm M1421 mm M1624 mm M1827 mm M2030 mm

The rest of the job at this size

Tap drill for M4 × 0.73.3 mm Tightening torque for M4 × 0.7every grade

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