How Rain Barrels Work

Water Harvesting

A rain barrel can cos­t as little as $20.00 to make, or it can involve a sophisticated system of pumps and tanks that can run into thousands of dollars. Luckily, raindrops add up quickly, so you don't have to make a major investment to reap the benefits of capturing rainwater from the roof of your home. Let's take a look at a few numbers. One inch (2.54 centimeters) of rain falling on a 1,000 square foot (93 square meter) roof will yield 600 gallons of water (about 2,271 liters) [source: Healthy Landscapes]. That's quite a bit of water. The roof itself is your water catchment area, and the water running from your roof down your gutters is fed directly to your rain barrel.

How much can you expect to harvest from your roof? This simple formula will help you figure your potential yield, less any water you may lose from evaporation or spills:


  1. Measure the exterior length and width of your home, plus any roof overhang.
  2. Now multiply the two measurements together. A house that's 30 feet by 40 feet (around 9 by 12 meters) will have 1,200 square feet (almost 111.5 square meters) of roof catchment area.
  3. Now, multiply 1,200 by 0.6, the portion of a gallon that will be harvested by one inch of water falling on one square foot (0.09 square meter) of your roof's surface.
  4. Your 1,200 square foot roof catchment area will yield 720 gallons (a little more than 2,725 liters) of water from one inch (2.54 centimeters) of rainfall (30 x 40 = 1200 x .6).

That's pretty impressive. If you take a look at the difference in your water bills between the summer and winter months, you can start to understand how capturing rainwater can save you money. The average American household uses 30 percent of its total water for outdoor applications [source: EPA].

In the next section, we'll talk about what you need to consider when installing a rain barrel.