The standard bathtub, which measures 60 inches long by 30 inches wide, holds approximately 35 to 50 gallons of water. Larger units, such as soaking tubs or freestanding tubs, hold approximately 60 to 80 gallons of water. Spa or corner tubs typically hold 70 to 120 gallons of water.

By comparison, a 10-minute shower consumes approximately 25 to 50 gallons of water or more, depending on the type of showerhead used. The maximum volume of a bathtub is calculated by multiplying the inner width, length and depth in inches and then dividing the product by 1,728 to convert it into cubic feet. The tub's volume, in gallons, is then determined by multiplying the total number of cubic feet by 7.5, which is the number of gallons in 1 cubic foot.