A device (100, 100′, 200, 300) for filling gas storage tanks (10) comprises a frame (110, 210, 310) and/or a housing, a gas buffer storage tank (120, 220, 320) for a gas which is movably arranged with respect to the frame and/or with respect to the housing, a first valve (130, 230, 330, 331) comprising an inlet, connected to the gas buffer storage tank (120, 220, 320), and an outlet for the connection to a gas storage tank to be filled, and a weighing device (140, 240, 340), connected to the gas buffer storage tank (120, 220, 320), for weighing the gas buffer storage tank (120, 220, 320).