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Here are some commonly accepted values for sheet metal thickness in North America.
This table is for reference only and it is highly recommended that you check with a local supplier to establish what actual thickness values are used in your particular location.
Sheet metal thickness gauges for steel are based on a weight of 41.82 pounds per square foot per inch of thickness. This is known as the Manufacturers' Standard Gage for Sheet Steel. For other materials such as Aluminum and Brass the thicknesses will be different. Thus a 10 gauge steel sheet which has a thickness of 0.1345 inch will weigh 41.82*0.1345 = 5.625 pounds per square foot. Examples: 16 ga CRS is 2.5 pounds per square foot. For 18 ga CRS the weight is 2.0 pounds per square foot and for 20 ga CRS the weight is 1.5 pounds per square foot.
Thickness is expressed in inches.
Gauge Nominal Value for Steel Carbon Steel Galvanized Steel Stainless Steel Aluminum
08 0.165 0.1644 0.1681 0.1719 0.1285
09 0.150 0.1495 0.1532 0.1563 0.1144
10 0.135 0.1345 0.1382 0.1406 0.1019
11 0.120 0.1196 0.1233 0.1250 0.0907
12 0.105 0.1046 0.1084 0.1094 0.0808
14 0.075 0.0747 0.0785 0.0781 0.0641
16 0.060 0.0598 0.0635 0.0625 0.0508
18 0.047 0.0478 0.0516 0.0500 0.0403
20 0.036 0.0359 0.0396 0.0375 0.0320
http://en.wikipedia.org/wiki/Sheet_metal_gauges