Pressure of a tank will vary depending on the type of liquid or gas in the tank and where in the tank you want to know the pressure. Typically, the calculation is at the bottom of a tank. To perform the calculation, you need to know the density of your liquid or gas and the height at which you are making the calculation in relation to the tank. The calculation requires the formula using 32.17 feet per second squared -- the acceleration from Earth's gravity.
Multiply the density of the liquid or gas by how deep in the tank you want to calculate the pressure. For example, assume you have water with a density of 60 lb. mass per square foot (lbm/ft²) and you want to find the pressure 10 feet down the tank. So, 60 lbm/ft² times 10 feet equals 600 lbf/ft².
Divide 32.17 feet per second squared by 32.17 lbm-ft per lbf-second squared. The result is 1 foot squared per 144 inches squared.
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Multiply the result from Step 1 by 1 foot squared per 144 inches squared. In the example, 600 lbf per ft squared times 1 foot squared per 144 inches squared equals 4.167 lbf per inch squared.