Identify the material--e.g., plastic or metal--for which the film thickness conversion is required.
Classify the metal type as steel, galvanized steel, stainless steel, aluminum, copper or the appropriate alloy. Plastic need not be categorized, as gauge number scales are the same for all plastic types.
Take a minimum of three thickness measurements at various points on the material sheet, using the screw micrometer, and calculate the average thickness. Report the thickness in micron.
Multiply the plastic film thickness measured in micron by the conversion factor 3.937.
Round the result to three decimal places.
Report the result as a number followed by the word "gauge" to differentiate it from other commonly used units.
Convert the micron measurement to inches, as most sheet metal gauge tables only correlate thickness measurements given in inches with gauge numbers. Multiply the micron measurement by 3.937E-05 to obtain the equivalent measurement in inches.
Consult the Gauge and Weight Chart for Metals provided by the Engineering Toolbox or an equivalent gauge chart and isolate the sheet thickness column associated with the relevant metal--e.g., galvanized steel or aluminum. Gauge numbers for metals are based on the weight or density of the sheet metal, so the same gauge number refers to different thicknesses for different alloys or metal types.
Find the value in the table which is closest in magnitude to the thickness--in inches--requiring conversion, and read off the equivalent gauge thickness.