Open Internet Explorer, Firefox or any other web browser on your computer to navigate to the Gauge and Weight Chart. The link for this chart is below in the Resource section.
Find the thickness in inches in the column "US Standard Gauge" that corresponds to a given gauge value; for example, the gauge 30 corresponds to 0.0125 inches.
Multiply the thickness in inches by 25.4 to convert the gauge to millimeters. In our example for the gauge 30 the thickness is 0.0125 x 25.4 = 0.3175 millimeters.