The typical bathtub holds 80 to 110 gallons of water depending on its depth and measures 60 inches long and 30 inches wide on the outside, according to SF Gate Home Guides. The capacity of any bathtub is determined by taking measurements and multiplying the inside length, width and depth.
The depth of the bathtub should be measured from the bottom of the bathtub to the bottom of the overflow drain hole. The result of the multiplication is then divided by 1,728 to determine the tub’s capacity in cubic feet. This figure is then multiplied by 7.5 to arrive at the bathtub's capacity in gallons.