The average corner tub is 54 by 54 inches, with a diagonal of 72 inches, and if that tub has a depth of 14 inches, its capacity is 88 gallons. With an overflow drain placement 6 inches below the top of the tub, the capacity is a mere 50 gallons. Freestanding tubs come in many designs to fit a variety of spaces, and they tend to be smaller and deeper than inset tubs, but they hold more water.