When working with sheet metal, the term “gauge” is often used. Gague are used to specify the thickness of a metal sheet.
Gague (Ga.) is a length measurement unit for diameters originating in North America and belongs to the Browne & Sharpe metering system. Originally used in the fields of medicine and jewellery, the larger the number, the smaller the diameter, and now it is also used to indicate the thickness.
Ga. is different from inches, there is no conversion formula. Even when the non-ferrous metal plate and the steel plate are the same Ga., the thickness is actually different.
You can find the gauge to mm / inch conversion for sheet metal by the chart below.