The level of a tank of liquid is the height of the liquid, or how much liquid there is in the tank. If you try to measure by eye you can miscalculate the amount, whether because of human error or because there is a dip in the surface of the liquid, called the meniscus. To compensate for that, you must calculate tank level using pressure.
Determine the specific gravity of the liquid you are dealing with. If it is water, the specific gravity is 1.00. If it is another liquid, visit the SI website in the Resources section to find out its specific gravity.
Read the measurement for the pressure of the liquid. You will need an accurate pressure reading to determine the tank level.
Convert the pressure from pounds per square inch (PSI) into inches of water (inH2O). One PSI is equal to 27.71 inches of water, so multiply the PSI by 27.71. For example, if the PSI is three, you multiply three by 27.71, which equals 83.13 inH2O. ("Inches of water" is a measurement, not necessarily a description of the liquid you're measuring.)
Calculate the tank level from the inH2O. Divide the inH2O by the specific gravity of the liquid. Our example uses water at 3 PSI, which is 83.13 inH2O. Divide 83.13 by 1.00, because 1.00 is the specific gravity of water. The result of the equation is 83.13 inches. That is the height of the tank level.