Is 12 gauge 12 mm

No, 12 gauge does not equal 12 mm—the gauge system and metric measurements use completely different scales. For wire gauge (AWG - American Wire Gauge), 12 gauge wire has a diameter of approximately 2.053 mm, far smaller than 12 mm. For sheet metal gauge, 12 gauge steel thickness is approximately 2.66 mm (0.1046 inches). For shotgun bore gauge, 12 gauge refers to bore diameter approximately 18.5 mm (0.729 inches), which is larger than 12 mm but still not equivalent.

The gauge numbering system originated from how many lead balls of bore diameter (for shotguns) or how many wire drawing passes (for wire) were needed to achieve specific dimensions, creating an inverse relationship where higher gauge numbers indicate smaller dimensions for wire and sheet metal. This creates significant confusion when people assume gauge numbers correspond directly to millimeter measurements. When specifications mention "gauge," always clarify which gauge standard applies (AWG for wire, manufacturer's standard for sheet metal which varies, bore designation for shotguns) and consult conversion tables for the actual dimensions in millimeters or inches. For wire and sheet metal work, relying on gauge numbers without understanding their actual dimensions leads to selection errors—always verify the actual thickness or diameter in millimeters or inches before proceeding with fabrication, assembly, or procurement to ensure compatibility with design requirements.