The gauge of a sheet of metal is a reference to how thick it is. The higher the numbered gauge a piece of metal is, the thicker the piece is in millimeters. If you want to find out what gauge your sheet metal is, measure its thickness using a regular tape measure. Then, using a sheet metal gauge chart, you can match that thickness up with the appropriate numbered gauge of your product.
Use a tape measure to find the thickness of your sheet metal piece. Use the millimeter hash marks to find the most accurate measurement possible.
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Multiply the number of millimeters by 0.03937 to convert to inches. If your measurement was 55 millimeters, for example, you would be left with 2.16535 inches.
Compare the thickness of your sheet metal in inches to a sheet metal gauge chart to find the proper gauge of that particular piece. For example, if your sheet metal is 0.2391 inches, it would have a gauge of 3.