Rain barrels provide a way to balance watering needs during the summer, when weather is more sporadic. Rainfall is unpredictable and hot dry spells at times seem to last forever. Most storms deliver 1/4 inch of rain, but 5 to 6 times per year we get 1-inch deluge. Because the typical residential roof can collect a significant amount of rainfall, the amount of water saved depends on the capacity of your rain barrel(s). If you want to store water for irrigation between summer storms, you will need a rain barrel of adequate size. Connecting 2 or 3 barrels together or installing a barrel at more than one downspout is an easy way to multiply your storage capacity.

So after you consider how much water your garden needs, calculate how much water you will get from your roof. For example: 1 inch of rain on 1 square foot of roof yields approximately 9.6 gallons or 2.3 liters. Therefore, 1 inch of rain onto a 10 ft x 10 ft area (100 sq ft) yields 60 gallons or 227 litres.

Multiply the square footage of your roof that drains into each individual downspout by 0.6 gallons / 2.3 liters to get your roof runoff volume quickly and easily by using our custom made calculator above. Just round off the area of your roof to the nearest square foot, enter the value into the input box and press "calculate". We'll figure out your approximate roof runoff volume in both liters and gallons!