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. This is known as the Manufacturers' Standard Gage for Sheet Steel. For other materials, such as aluminum and brass, the thicknesses will be different. Thus, a 10 gauge steel sheet which has a thickness of 0.1345 inches will weigh 41.82*0.1345 = 5.625 pounds per square foot.
Examples: 16 ga CRS is 2.5 pounds per square foot. For 18 ga CRS the weight is 2.0 pounds per square foot and for 20 ga CRS the weight is 1.5 pounds per square foot.
Standard sheet metal gauges
GaugeSteelCarbon Steel ehGalvanized SteelStainless SteelAluminumSteel (mm)070.179----4.547080.1650.16440.16810.17190.12854.191090.1500.14950.15320.15630.11443.810100.1350.13450.13820.14060.10193.429110.1200.11960.12330.12500.09073.048120.1050.10460.10840.10940.08082.677130.09--0.0940.0722.286140.0750.07470.07850.07810.06411.905150.067--0.070.0571.702160.0600.05980.06350.06250.05081.524170.054--0.0560.0451.372180.0470.04780.05160.05000.04031.1938190.042--0.0440.0361.067200.0360.03590.03960.03750.03200.9144210.033--0.0340.0280.838220.03--0.0310.0250.762230.027--0.0280.0230.686240.024--0.0250.020.61250.021--0.0220.0180.533260.018--0.0190.0170.457270.016--0.0170.0140.406280.015--0.016-0.381290.014--0.014-0.356300.012--0.013-0.30531---0.011--