In one aspect of the present invention, an apparatus for measuring the height of liquid contained in a storage tank is disclosed. A tube is disposed inside the tank. A float is disposed inside the tube and is buoyed on the surface of the liquid. An ultrasonic transducer is disposed inside the tube. The ultrasonic transducer emits ultrasonic pulses toward the float, receives the reflected ultrasonic pulses, and responsively produces an echo signal. The float has a top portion and a bottom portion separated by a cylindrical portion. The bottom portion including an spherical surface which receives the ultrasonic pulses. The spherical surface has a predetermined radius which is a function of the inside diameter of the tube, the height of the cylindrical portion of the float and the outside diameter of the cylindrical portion of the float. A temperature sensor monitors the temperature of the liquid and produces a thermometric signal in response to the liquid temperature. A microprocessor receives the echo and thermometric signals, determines the speed of the ultrasonic pulse traveling in the liquid, and responsively determines the liquid height.