A gauge of three, the lowest for most standard sheet metals, represents one-quarter-inch thick (6.35 millimeters) stainless steel, or 0.23 inch (5.82 millimeters) thick aluminum, while a gauge of 36 equals 0.007 inch (0.18 millimeter) thick stainless steel. Higher gauges indicate thinner sheet metal.
The gauge, also spelled gage, is a set standard for the thickness and weight of sheet metals. Gauges differ between iron and iron-based metals like steel and other metals like aluminum or brass. A sheet of steel is slightly thicker than a sheet of aluminum of the same gauge, while the same gauge of zinc is much thinner than either. Zinc sheet metal of a gauge of three is only 0.006 inches (0.15 millimeter) thick. The lowest gauges, zero to 0000000, are only used for heavy steel sheets and iron plates between 0.31 to 0.50 inch thick, while the highest standard gauge of 38 refers to steel that is 0.006 inches (0.15 millimeter) thick.
According to standards organizations such as the American Society for Testing and Materials, designating sheet metal thickness by gauge is an outdated and discouraged system, as different manufacturers have different standards for how many decimal points to use when calculating the gauge. For this reason, manufacturers now prefer to define sheet metal thicknesses by exact measurements in millimeters.