What wrench fits a M4 bolt?

Across-flats wrench/socket size by head type - a flange bolt often uses a smaller wrench than a standard hex bolt of the same diameter, so always check the head type.

System

Bolt size

Nominal diameter	4 mm
Standard hex	7 mm
Hex flange	not commonly available in this head type
Recommended clearance hole (medium fit)	4.5 mm
≈	≈ 7/16" (1/4")

Reference values only - always verify against the actual part before machining, tapping or tightening anything.