What wrench fits a M18 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	18 mm
Standard hex	27 mm
Hex flange	24 mm
Recommended clearance hole (medium fit)	20 mm
≈	≈ 1-1/8" (3/4")

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