Gage vs. Gauge

For thicknesses, the preferred spelling is "gage" - use "gauge" for a temperature gauge, etc.

Gages are thicknesses (sheet metal) and diameters (metal wire). Standard gages are round numbers like 24, 26, 28, etc. - the larger the number, the thinner the material. 26 gage sheet metal is bendable by hand. Steel door frames are typically made out of 18 gage. Gages are legacy inch-pound dimensions that have not gone out of use even though metal industry recommendations are to use inches and mm. There were/are several different gage standards, but the only ones still in use are for steel sheet and steel wire.

Gages are not computable - e.g. 12 gage is not twice the thickness of 24 gage.

All metals should be periodically looked up in the relevant ASTM standards to see whether the standards define standard thicknesses. When last checked, the steel sheet metal standards did NOT define standard thicknesses, although they do define thickness tolerances.

Steel Sheet and Wire: Shared Attributes has the gage charts. To convert a gage dimension into metric, first determine its decimal inch thickness from the chart and then convert that to mm. When stating a gage in the spec use this format:

Other Metals: For sheet metal other than steel, such as aluminum, copper, zinc, etc., use decimal inch thicknesses. Copper sometimes is expressed as xx pounds per square. A square is 100 square feet - a roofer’s measurement (sheet copper was mostly used on roofs).