The industry convention of using gauge to denote the thickness of steel sheets (as opposed to the actual measurement in inches) allows manufacturers to express the cost of sheeting in terms of raw material usage. Known as the "Manufacture's Standard Gauge For Sheet Steel" (MSG), the system uses the weight of a 12" by 12" by 1" piece of steel (i.e. 41.82 pounds per square foot) as a baseline. While a formula originally existed for directly converting gauge number into actual thickness, manufacturers soon realized that the density of solid steel is significantly lower at the surface (a phenomenon called "crowning"). For example, a cubic foot composed of twelve 12" by 12" by 1" steel plates would weigh less than a cubic foot of solid steel. Today's MSG system is basically a version of the earlier gauge measurement that's been recalibrated to reflect the effects of crowning. Therefore, the most accurate way to convert steel gauge to thickness is to simply consult the official MSG definitions.
Determine which specific type of steel the sheet is made of, e.g. sheet steel, galvanized steel or stainless steel.
Determine whether the stated gauge of the sheet follows the "U.S. Standard Gauge" system or the "Manufacturer's Standard Gauge" system. Note: the largest thickness of sheet steel in the MSG system is "3." If the stated gauge is 2, 1, 0, 2/0, 3/0, 4/0, 5/0, 6/0 or 7/0, then the U.S. Standard System is being used. Similarly, the largest thickness of galvanized steel under MSG is "8" while the largest MSG thickness for stainless steel is "6/0."
Visit "http://www.engineeringtoolbox.com/gauge-sheet-d_915.html" to look up the official thickness (in inches) of your material's specific gauge.
Multiply this measurement by 25.4 to convert its units to millimeters.