A typical bathtub contains between 35 and 50 gallons of water and is 60 inches length by 30 inches wide. Larger fixtures, including soaking tubs or freestanding tubs, have a capacity of 60 to 80 gallons. Spa or corner baths typically have a capacity of 70 to 120 gallons.

Comparatively, depending on the showerhead used, a 10-minute shower can use anywhere from 25 to 50 gallons of water or more. By multiplying the inner width, length, and depth in inches and dividing the result by 1,728 to convert to cubic feet, the maximum volume of a bathtub can be determined. The total number of cubic feet is then multiplied by 7.5, which is how many gallons there are in a cubic foot, to get the tub’s volume in gallons.