Volume of a fluid, such as gasoline or diesel fuel, in a tank is determined by measuring the pressure of the fluid using a pressure sensor positioned proximate the bottom of the tank. The depth of the fluid in the tank is then calculated by dividing the pressure by the density of the fluid. Fluid volume is then determined mathematically or from charts given the depth as well as the size and shape of the tank. Multiple pressure readings may be taken along or near the bottom of a tank, and an average pressure determined that may be used to calculate measured volume. To maintain accuracy at different altitudes, pressure readings are preferably adjusted for atmospheric pressure using differential pressure sensors or a processor using data indicative of both pressures. Volume changes exceeding a predetermined threshold, or which are not comparable to dispensed fuel, may be flagged and alerts generated.