A gauge conversion chart can be used to determine the actual thickness of sheet metal in inches or millimeters. For example, 18 gauge steel, according to a gauge conversion chart, is 0.0478 inch or 1.214 millimeter.

Sheet Metal Thickness (Gauge) Chart in Inches Gauge Mild Steel Aluminum Galvanized Steel Stainless Steel 3 0.2391 0.2294 0.2500 4 0.2242 0.2043 0.2344