To determine the thickness of a metal gauge, determine what kind of metal you have, and use a standard sizing chart to determine the thickness. If you do not know the gauge, use a measuring tool.
The thickness of any given gauge varies depending on the specific metal. Metals are generally separated into ferrous and non-ferrous types, although the thickness of a gauge can vary within those categories as well. Sheet metal is generally manufactured according to a standardized scale known as the Manufacturers' Standard Gauge for Sheet Steel, while wire is measured using the American Wire Gauge in the U.S. and Canada and the Standard Wire Gauge in the United Kingdom. Thus, if you know the gauge of the metal, you can generally determine the thickness without measuring it.
However, if you are unsure of the gauge or need a specific measurement, the easiest way to do that is to use a special measuring tool, which is also known as a gauge. Gauges are circular and feature notches of various sizes around the edge. To use them, insert the edge of the sheet metal or end of the wire into the notches until you find the one that fits it perfectly. The numbers on the gauge should give you an accurate measurement of thickness. This is more reliable than using a ruler because of the thinness of the metal or wire.