Sheet metal thickness gages are based on a weight of 41.82 pounds per square foot per inch of thickness. This is known as the Manufacturers' Standard Gage for Sheet Steel, and is primarily used for sheet steel. For materials such as Aluminum and Brass the thicknesses will be different. Thus a 10 gage steel sheet which has a thickness of 0.1345 inch will weigh 41.82*0.1345 = 5.625 pounds per square foot.
In the U.S the most common sheet metal gage specified is the Steel Gage. In the U.K. the most common sheet metal gage is the British Standard Wire Gage. Black iron, steel sheet, and hooping are specified by the Birmingham Gage,
which is different from the Birmingham or Stub's Iron Gage that is used for wire gages.