Sheet steel is steel formed into thin flat pieces which come in sheets or in roles and is used for metalworking. Steel sheets can be made out of cold-rolled steel, galvanized steel and stainless steel. Standard sheet steel comes in various thicknesses which are classified by steel gauge. Each gauge has a tolerance range to allow for a small error in thickness. A higher steel gauge value means the material will be thinner and a lower gauge signifies a thicker steel piece. For example, a piece of standard steel with a gauge of three is 0.2319 inches thick while a piece with a steel gauge of 23 will be 0.0269 inches thick. By knowing the weight of the gauge of steel you are working with, you can determine the thickness.

Determine what type of steel sheet you are working with. Cold-rolled steel sheets do not have any coating or chemical additives. A galvanized steel sheet contains a 0.0010-inch zinc coating. Stainless steel sheets are a mixture of steel with chromium which allows it to be corrosion-resistant. The thickness will vary depending on the type of steel you are measuring.

Cut a one-square-foot piece (that is, one-foot by one-foot) out of the steel sheet for which you would like to calculate the thickness. This size of the sheet will help make the calculation easier to work with. You can weigh a larger steel piece but you will have to account for the extra area in your equation.

Obtain the weight of the piece of steel you are working with in pounds per square foot. The gauge number and weight of the steel are directly related. The weight is also a necessity in calculating the thickness of the steel. A steel-thickness gauge is based off of the Manufacturer's Standard Gauge for Sheet Steel, or the weight of a one-inch thick piece of steel, which is 41.82 pounds per square foot per inch of thickness.

Write down the following equation: Weight of steel you are using in pounds per square foot divided by the Manufacturer's Standard Gauge for Sheet Steel in pounds per square foot = Gauge thickness of a steel sheet in gauge decimal (inches) or theoretical decimal thickness.

Solve the equation with a calculator using the weight of steel per square foot you just measured. For example, you know that you have a one-foot-square piece of eight-gauge standard steel. You weighed the steel piece and found it to be 6.875 pounds. Plug this weight into your equation and solve: (6.875 pounds per square foot) divided by 41.82 pounds per square foot = 0.1644 gauge decimal (inches).