It takes approximately 24 gallons to fill a standard American bathtub to the brim. Larger bathtubs, such as jetted tubs and garden tubs, can hold between 45 and 80 gallons of water.
A custom bathtub may hold up to four times as much water as a standard tub, depending on the dimensions of the tub and the space that items, such as arm rests, take up in the basin. A person in a standard sized bathtub may only use 15 gallons of water or less when bathing. Ensure safety when bathing toddlers by only filling the tub to only about 2 inches deep.