Sheet metal thickness is denoted by gauge, sometimes spelled gage, which indicates a standard thickness before processing.  Click here for a Gauge to mm Conversion Chart.  Processing may include polishing, or the applying of protective plastics which will decrease or increase a sheet’s thickness respectively.  As the gauge number increases the material’s thickness decreases.  Below is our best attempt to capture all of the standard gauge thickness of different materials.  Be sure to check with your supplier when purchasing steel what your actual thickness is going to be, especially if you’re receiving polished or treated material.