Abstract: A method for determining the level of fuel in tanks otherwise un-modified is provided. The method is based on the variation of the acoustic frequency response of the tank in the sonic range with the fuel level in the tank. In a preferred embodiment, the tank is a reservoir for propane used as fuel for preparing food. The operator, such as a homeowner, attaches the tank to his/her grill. The operator engages the proposed new art by tapping the tank and receiving the acoustic signal emanating from the tank with a microphone such as one embedded in a smartphone. The smartphone examines a cluster of peaks in the sonic frequency spectrum between 0 Hz and 20 kHz. By capturing the frequency of the first large peak above 900 Hz in the frequency spectrum, the smartphone converts the frequency into an estimate of the amount of fuel remaining in the tank.