Abstract: An apparatus is provided for ascertaining whether or not the contents of an aerosol medication inhaler canister are near depletion. The apparatus is comprised of a tank having transparent walls and a reference indicia on the tank visible through a transparent front wall panel. The reference indicia depicts the orientation of the longitudinal axis of an empty aerosol medication inhaler canister floating in water relative to horizontal. Preferably, the reference indicia is mounted on a dial attached to the rear wall of the tank. The dial includes a pointer. Canister model code marks are printed on the rear wall of the tank immediately beneath the dial. The user rotates the dial to align the pointer with the particular model code mark corresponding to the canister model the user wishes to measure. This brings the reference indicia on the dial into the fully discharged orientation which an empty canister of that model will assume when placed in water.