Further, to tell our general aviation friends how extremely accurate the gages are in an airliner...
If you have a "float" quantity indicator (like in a car, or a Cessna)... the gage, if accurate, measures merely the VOLUME of the fuel in the tank... That is not good enough for airlines. With the capacitance probes used in airliners, you actually read the WEIGHT of the fuel, regardless of its density, and temperature. You all know that volume changes with temperature, because of density...
Jet engines are only interessed in burning kilos or pounds of fuel, not liters or gallons...
In addition, there are generally two probes (or more) per tank, so as to take an average and accurate reading regardless of attitude of the airplane.