A sheet metal gauge/gage is an indication of the standard thickness of specific sheet metal materials, such as steel, aluminum, and copper. The larger the gauge number, the thinner the metal (not applicable for zinc).

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.

A sheet metal gauge (sometimes spelled "gage") indicates the standard thickness of sheet metal for a specific material. As the gauge number increases, the material thickness decreases. Sheet metal thickness gauges for steel are based on a weight of 41.82 pounds per square foot per inch of thickness.

In the U.S., the thickness of sheet metal is commonly specified by a traditional, non-linear measure known as its gauge. The larger the gauge number, the thinner the metal. Commonly used steel sheet metal ranges from 30 gauge to about 7 gauge.

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.