Filling units for filling small tanks or the like with gasoline, kerosene, or other flowable materials which attach an inlet to such a tank. When the filling unit is in a fill position, a displacer section occupies a substantial volume within the tank. It allows the person filling the tank to supply the liquid until the tank will accept no more and the liquid reaches a predetermined level in a funnel portion of the filler. Thereafter, removal of the displacer section from the tank, as by sliding a section of the overall unit upward, allows the liquid in the funnel to enter the tank and fill the volume evacuated by the displacer section to complete the filling without any spillage. The overall unit may be removed from the tank each filling operation, or it may be designed to include a protective cap that fits over the upward-protruding portion of the unit, allowing the unit to remain mounted on the tank.