An apparatus and method are provided for calibrating manifolded liquid storage tanks. The apparatus includes sensors disposed within the tanks for sensing the liquid levels in each of the tanks and a metering device for measuring the amount of liquid dispensed into or out of the storage tanks. A processor is also provided which determines the amount of liquid dispensed from each tank based on liquid level measurements obtained during idle periods following dispensing periods. Once this information is obtained, single tank calibration techniques are employed. An alternate combined tank calibration can also be used. This method does not separate out the amount of liquid dispensed from each tank, but rather utilizes the combined amount dispensed. From this simpler, but less accurate method, an "equivalent" combined tank chart or combined mathematical formula may be generated using the metered amounts and a combined height amount. Heights from the two tanks may be simply averaged. The processor may be programmed to perform either or both methods.