Calculate sheet metal thickness by using a sheet metal gauge chart. Select the appropriate chart based on the type of sheet metal you have, find the gauge of the sheet metal on the chart and then locate the corresponding thickness on that gauge's row of the chart. For example, a 10-gauge sheet of standard structural steel corresponds to a thickness of 0.1345 inches or 3.416 millimeters.
Selecting the correct chart for the type of metal you have is essential for finding the correct sheet metal thickness, as gauge standards vary considerably over different types of metal and sometimes even between different variants of the same metal. Steel, for example, has different gauge standards for regular structural, galvanized and stainless steels.
Aluminum, brass and other metals and alloys that do not contain iron usually use the American Wire Gauge system in the United States, while the Birmingham Gauge is more common in the United Kingdom and its territories.
Sheet zinc uses a completely unique system not shared by any other metals. Most gauge systems have an inverse relationship between gauge and thickness, meaning a higher gauge indicates a thinner sheet. Zinc gauge standards, on the other hand, use a direct correlation between gauge and thickness, meaning that higher gauges correspond to greater thicknesses.