The amount of water in a bottle can vary based on its size. The three most common bottle sizes are 16.9 oz, 20 oz, and 33.8 oz. Knowing that there are 128 ounces per gallon, a 16.9-ounce bottle holds about 0.13 gallons, a 20-ounce bottle holds about 0.16 gallons, and a 33.8-ounce bottle holds about 0.26 gallons.