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Understanding how much water your rain water harvesting system will catch is very important in deciding how big of a tank you want to install and how much water you will have captured during the dry season. The amount of rain a gutter or downspout can collect is determined by the area of the roof the gutter is attached to.
Here is the formula for calculating how much rainwater will be collected in your tank from an inch of rainfall
Here we will walk you through an easy way to find the square footage of your roof and use the equation above
The first step to calculating your rain water harvesting (RWH) potential is to find the square footage of your roof. This can be done by simply measuring your roof or using Google Earth.
Locate your roof on Google Earth
Click the "ruler" icon on the top of the screen
Click on the corners of your roof to measure the area
Its important to think of how water will flow on your roof. If you have a sloped roof, only measure the area of the part of the roof that will slope into the downspout you intend to install a rain tank under.
Example:
On this slanted roof at the Village Garden at UCSC we want to install a rain tank under the bottom left downspout so we highlighted the side of the roof sloping into that corresponding gutter. On the right of the image below you can see that our roof area is approximately 1100 square feet. Its okay to be off by a couple feet.
The second step to calculating your roof's RWH potential is to multiply this square footage by the conversion factor of 0.623 gallons of water per inch of rainfall per square foot of roof area.
Example
The third step to calculating your roof's RWH potential is to locate and divide by the number of downspouts on the side of the roof you have measured. Unless you block or reroute any downspouts, an equal amount of the runoff from your roof will go to each downspout.
Example
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