Quick Answer
To estimate the amount of rainwater you can collect, calculate your roof's catchment area in square feet, multiply it by the average annual rainfall in your area, and then account for gutter system losses and efficiency.
Calculating Roof Catchment Area
To calculate the catchment area, start by measuring the length and width of your roof in feet. Multiply these two numbers together to get the total square footage. For example, if your roof is 30 feet long and 40 feet wide, the catchment area would be 30 x 40 = 1200 square feet.
Accounting for Rainfall and Gutter Efficiency
Next, research the average annual rainfall in your area. In the US, this information is often reported in inches and can be found through government websites or local weather services. For example, let’s say the average annual rainfall in your area is 40 inches. To calculate the total amount of rainwater you can collect, multiply the catchment area by the rainfall in inches, then convert the result to gallons. There are 7.48 gallons in a cubic foot, so multiply the result by 7.48. For a 1200 square foot roof with 40 inches of annual rainfall, the calculation would be 1200 x 40 x 7.48 = 364,160 gallons.
Gutter System Losses and Efficiency
However, not all of this rainwater will make it through your gutter system. You’ll lose some to gutter overflow, dirt and debris, and system inefficiencies. A good rule of thumb is to account for 10-20% losses due to gutter system inefficiencies. In our example, this would mean subtracting 36,416 to 72,832 gallons from our total, leaving around 327,328 to 291,328 gallons collected annually.
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