3 Sizing of Sheet Metal
The thickness of sheet metal is referred to as gauge. In a gauge system, the smaller the number, the thicker the material. 
Gauge systems typically use only even numbers with a few exceptions, like 11Ga. Sheets can be measured easily with a sheet gauge.
Sheet gauges are specific to the type of metal being measured and all have a slight difference. A manufactures standard gauge (MSG) is used for bare steel, U.S. Stand gauge (USG) is used for stainless steel, a Galvanized Sheet Gauge (GSG) is used for any coated steel and a non-ferrous or Brown and Sharpe (B&S) is used on aluminum and other non-ferrous metals.
Some metals don’t use gauge system. Instead they use weight. Copper and brass use ounces per square foot (oz/sq ft). Lead uses pounds per square foot (lbs/sq ft).
Because of so many different ways to measure sheet metal, today it is common to use decimals of an inch to size all metals. The use of decimals can eliminate some of the errors caused by using an incorrect sheet gauge.
Micrometers or caliper are how we measure such small thicknesses.
Metals or alloys that contain less than 5% iron and in most cases, contain no iron (i.e., aluminum, copper, zinc, and lead). Examples of non-ferrous alloys are bronze and brass.
