We have created the rainwater collection calculator below to assist you with understanding the rainwater collection opportunity you have and to assist you in sizing an optimal rainwater collection tank size for your situation.

In the calculator below, choose your state and then choose your city.  If you city isn’t listed, please choose a city that is closest to you.

Then you need to enter the amount of roof area that you will be collecting from.  If you don’t know how to calculate the roof collection area, see the explanation section below the calculator.

After you hit “Calculate”, you will be presented with the Total Annual Rainwater Collection Volume, in gallons, along with a graph showing the amount of rainwater you would collected on a monthly basis.  These monthly volumes are calculated using the monthly average rainfall amounts for your area.

Try it for yourself!

The NCEI 30-yr Normal Monthly Rainfall (1981-2010) dataset is the source of the location specific rainfall data.

To calculate the area of your roof, simply multiply the length (in feet) by width (in feet) for square or rectangular roofs. The resulting figure represents the surface area of your roof in square feet.

You don’t need to worry about the slope of your various roof plans.   Most of the time, rain falls straight down and thus the angle or slope of your roof really doesn’t matter in this calculation.  See the image below to help visualize the roof collection area.