A delivery unit for extracting liquid additive from a tank can be mounted on the tank and includes a fill level sensor for measuring the fill level of liquid additive in the tank. The fill level sensor is set up to emit waves into an emission region of the tank, and the fill level can be measured by using a propagation time measurement of the waves reflected by a liquid surface and striking the fill level sensor again. At least one first reference surface extends at least partially into the emission region and is at a first distance from the fill level sensor. The at least one first reference surface is disposed on a separate calibration component mounted on an outer side of a housing of the delivery unit. A tank for storing liquid additive, a motor vehicle and a method for monitoring a fill level are also provided.