Seat dimensions are 15.25inD x 20in W. Vinyl grips on the tubing prevent marring the tub. Tub must be 22in wide. This measurement must be taken at the center of the tub, inside wall to inside wall, and measure halfway down the wall and across to the other side of the bathtub. Bath seats are designed for bathtub use. If you desire to use in a shower stall, you must insert the bath seat into the floor stand to prevent the bath seat from turning over.