Steel is an alloy consisting mostly of iron, with a carbon
content between 0.2% and 2.14% by weight (C:110–10Fe), depending on grade.
Steel sheet metal gauge (sometimes spelled "gage") indicates the standard
thickness of the sheet metal. As the gauge number increases, the
material thickness decreases. The gauge of sheet metal ranges from 35
gauge to about 6 gauge. Thicknesses can vary significantly, although
extremely thin thicknesses are considered foil or leaf, and pieces thicker
than 6 mm (0.25 in) are considered plate.