Abstract: The disclosure provides a method of automatically determining a filling level of a plastic bag fastened via an open end to a vertical post forming a support, the method comprising a step of detecting that the bag has reached a predetermined filling level by means of a first distance sensor positioned on the post, a step of measuring the distance of the bag relative to the post by means of a second distance sensor positioned on the post, step of detecting the presence of the bag fastened via an open end to the post by means of a third distance sensor positioned on the post, and a step of detecting a filling level on the basis of the measurements taken by the distance sensors. The disclosure also provides a unit for performing such a method.