The amount of rainwater you can collect depends on several factors, including the size and pitch of your roof, the annual rainfall in your area, and the efficiency of your rainwater collection system. A general estimate is that 0.6 gallons (2.27 liters) of water can be collected per square foot (0.09 square meters) of roof per inch (2.54 centimeters) of rainfall. Therefore, a larger roof surface area and higher rainfall will yield greater rainwater collection potential.