Rainwater Harvesting Calculator


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Last Updated: August 16, 2021

Not sure how big your rain harvesting barrels need to be?

This calculator gives you an estimate of how much water you will be able to collect during an average rainfall.

*If you are only collecting rain from some of your roof’s downspouts, only use the roof’s dimensions, which are feeding these downspouts. For example, if you are only planning to have a rain barrel on one side of the roof, only use half of the roof width in the calculator.

How This Calculator Works

A moderate rain shower that lasts 3-4 hours will produce approximately 1 inch of rain.  For each 1 inch of rain, a square foot of roof will capture 0.6 gallons of rainwater.

However, no rainwater system is 100% efficient at capturing all of the rain which falls.

A more realistic efficiency is 75% or 0.45 gallons per square foot.

Roof Length and Width

Because rain (mostly) falls straight down, your roof’s slope doesn’t matter.  What does matter is the length and width straight across. You can easily measure this from ground level.  Remember only to calculate the part of the roof which is feeding the gutters used to collect rain.

roof size calculator for rainwater harvesting

How Many Inches of Rain?

On average, a typical rainfall will produce 1 inch of rain.  Thus, 1 inch of rain is used as the default for figuring out how much rainwater can be harvested.  However, some areas get much more or less rain than this in a typical shower.

To get a more precise rainfall number for your area, you can do one of the following:

  • Go to the National Climatic Data Center and search for your regional program.
  • Find the average rainfall per each month for your area. Then divide this number by the number of rainy days for that month. *You shouldn’t use yearly precipitation amounts because amounts can vary drastically per month.  Precipitation amounts also include snow.
  • Put a rain gauge in your yard and measure how many inches of rain are produced during a rainfall.

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