A method for measuring the fill level of a liquid additive in a tank using an ultrasonic sensor includes initially carrying out a measurement of a first propagation time of an ultrasonic signal from the ultrasonic sensor to a liquid level of the liquid additive. This is followed by cleaning of at least one reference surface in the liquid additive with the aid of at least one ultrasonic pulse. Then at least one second propagation time of an ultrasonic signal from the ultrasonic sensor to the at least one reference surface is measured and the fill level is calculated from the first propagation time and the second propagation time. A delivery unit having at least one ultrasonic sensor for installation in a liquid additive tank is also provided.