A sheet metal gauge chart is a tool used to determine the thickness of sheet metal in either inches or millimeters. The thickness of sheet metal is represented by a number, with lower numbers indicating thicker sheets and higher numbers indicating thinner sheets. The most common sheet metal gauges used in the United States are the American Wire Gauge (AWG) and the Brown & Sharpe (B&S) gauge systems.

Here's an example of a sheet metal gauge chart in inches:

It's important to note that the thickness of a sheet of metal can vary slightly depending on the manufacturer and the specific type of metal being used. As such, it's always a good idea to verify the thickness of the metal using a micrometer or other measuring tool before beginning any fabrication work.