An amount of fuel in a fuel bladder of an unmanned aerial vehicle (UAV) is determined based on a deformation of the bladder. In some examples, a fuel system for a UAV includes a bladder, a proximity sensor, a fuel gauge, and a controller. The bladder is deformable based on the amount of fuel in the bladder. The proximity sensor may output a signal that varies as a function of the deformation of the bladder. The controller may determine the amount of fuel in the bladder based on the signal output by the proximity sensor and control the fuel gauge to indicate the determined amount of fuel.