Gauges are used to specify the thickness of a sheet metal. Gauges are neither standard nor metric and the values are independent of those measurement systems. A gauge conversion chart can be used to determine the actual thickness of sheet metal in inches or millimeters. Choose from our selection of sheet metal gauges in a wide range of styles and sizes. In stock and ready to ship. Choose from our selection of sheet metal gauges in a wide range of styles and sizes. In stock and ready to ship. In order to help you guys who specialized in metalworking industry to check sheet metal gauge to inch and sheet metal gauge to mm, I collect the sheet metal gauge sizes conversion chart (with inch and mm unit) to help you quickly find the sheet metal gauge thickness which including mild steel gauge thickness, aluminum gauge thickness ... Gauge (or gage) sizes are numbers that indicate the thickness of a piece of sheet metal, with a higher number referring to a thinner sheet. The equivalent thicknesses differ for each gauge size standard, which were developed based on the weight of the sheet for a given material.