Steel water tanks have uses in both residential and commercial applications. In residences, a tank may serve as a hot water appliance, while in commercial buildings, tanks may be used to store water as part of a hot water loop in the heating system, or for other purposes. The wall thickness is an important aspect of a water tank because it determines the amount of water pressure it can withstand.

Determine the weight of the tank. The weight is probably included in the manufacturer's specifications because it is relevant to structural concerns and the ease of installation. If it is not provided in the data, call the manufacturer and ask for the information. In the event that the manufacturer cannot provide a weight, place it on an industrial scale designed for heavy equipment.

Measure the dimensions of the water tank. Most water tanks are cylindrical because that shape handles pressure well and makes economical usage of material, but some tanks are rectangular. If cylindrical, measure the length and diameter of the tank. If it is rectangular, measure the width, length and height. It may be most convenient to measure in centimetres and then convert the measurements into meters.

Use the dimensions to calculate the surface area of the tank. The surface area of a cylinder is the area of the two ends, which are each equal to the diameter squared x pi over four (or .785), plus the surface area of the lengthwise section, which is pi (or 3.14) x diameter x length. The surface area of a rectangular solid equals the sum of its faces, or 2 x (length x width) plus 2 x (width x height) plus 2 x (length x height).

Look up the density of the tank's particular steel. If it is stainless steel, the density is approximately 8,000 kilograms per cubic meter.

Divide the mass of the tank by the quantity, surface area x density. This yields the thickness because the mass of the tank equals the density of material x the volume of material, and the volume of material equals thickness x surface area. It is important to use consistent units for this step; if you obtained the surface area and density in terms of meters and kilograms, make sure you convert the mass into kilograms before performing this calculation.