01 / Water
Rainwater harvesting calculator
Find out what one rainstorm can put in your barrel. Start with the roof area that drains to the downspout you plan to use.
One rain event
Estimated water reaching storage
The method
Measure the roof in plan view.
Use the horizontal footprint, then estimate how much of it drains to your barrel. A 1,000 ft² footprint split evenly between two downspouts gives each one about 500 ft².
In US units, multiply square feet by inches of rain and by 0.623. In metric, one millimeter of rain over one square meter is one liter. Apply your collection efficiency to estimate water reaching storage. Any volume above your available storage is overflow.
Collected roof water is for outdoor uses such as watering ornamental plants. Check local rules and route overflow away from the building. Seattle Public Utilities rainwater guide.