To determine how many hydro flasks in one gallon, you must first know the Hydro Flask size. If you have an 18-oz Hydro Flask, you would need about 7.11 of them to make a gallon. For a 32-ounce Hydro Flask, you would need 4 to make a gallon. For a 40-ounce Hydro Flask, you would need 3.2 of them.