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The Best Rainwater Harvesting Calculator
It Works with US or Metric Measurements

Use The Rainwater Harvesting Calculator To Figure Out The Size Of Your Storage System.

It can be a pain doing manual calculations for rainwater harvesting. With so many gardeners discovering the benefits of rainwater, we decided to come up with a better way. By entering a few numbers, our rainwater harvesting calculator does all the work for you.

It even calculates the roof area from your measurements! And whether you use the US or metric measuring system, our calculator has you covered.

How the Calculator Works

You need to know three things before calculating the amount of rainfall you’ll collect in one month:

  1. The area of the roof section that’s feeding the downspout.  Measure the length and width of that roof section in feet and multiply the numbers. The fancy term for the roof is the “rain catchment” area.
  2. The average monthly rainfall in your area for the month in question.  Oregon State University has a map of monthly rainfalls for each state. For Canada, use the information on the website Climates to Travel.
  3. The amount of water, in gallons, in a space that’s one square foot in area by 1 inch deep.  This number is a constant and stays the same with every calculation.

The Calculations for Rainwater Harvesting

The Square Footage of the Roof Area

For example, suppose we measure the roof section, and it’s 16 feet 5 inches long and 10 feet 6 inches wide.

First, convert the inches into decimal feet:

5 inches = 5/12 feet = 0.417 feet, so 16 feet 5 inches = 16.417 feet.

6 inches = 6/12 feet = 0.5 feet, so 10 feet 6 inches = 10.5 feet.

16.417 feet X 10.5 feet = 172.38 square feet for the roof area.

The calculator does all these conversions automatically based on the numbers you enter.

The Roof Area With the Metric System

The calculations are the same, but now you’re using meters and centimeters. To convert centimeters to decimal meters, divide them by 100.

How to Figure the Number of Gallons in 1 Square Foot, 1 Inch Deep

This section shows how to calculate the constant. 

One cubic foot of water = 7.481 gallons.

1 inch = 1/12 foot = 0.0833 feet.

So, 7.481 gallons/cubic foot X 0.0833 feet = 0.623 gallons in a space of 1 square foot by 1 inch deep.

For the metric system, we need to calculate the number of liters in 1 square meter, 1 centimeter deep

The math is the same as we use for US measurements, but now we’re using meters, centimeters, and liters.

One cubic meter of water = 1000 liters.

1 centimeter = 1/100 meters = .01 meters.

So, 1000 liters/cubic meter X .01 meters = 10 liters in a space of 1 square meter by 1 centimeter deep.

Putting this all together, the amount of monthly rainfall we could collect is:

US System: The area of the roof in square feet X the average monthly rainfall in inches X 0.623.

Metric System: The area of the roof in square meters X the average monthly rainfall in centimeters X 10.

An Example Problem

Let’s suppose it’s June, and you check the monthly rainfall map for your area.  You find that the monthly average for June is 6 inches of rain.  With a roof area of 172.38 square feet:

172.38 square feet X 6 inches of rain X 0.623 gallons/sq. ft. inch = 644.35 gallons of rainwater. If you wanted to catch all of it, that would take 6 – 12 rain barrels. That’s based on 50 – 100 gallon barrels.

That’s the average amount of rain you can expect to collect in June from your harvesting system.

Another way to look at this is a 1000-square-foot roof will catch 623 gallons of water if one inch of rain falls.

We hope our rainwater harvesting calculator makes it easier for you to plan your water storage.

Bob Styer

As a child, I hated gardening. That was mainly because Dad expected us to work in the garden every so often even though we thought play was more important. Over the years, though, I've developed a real appreciation for growing things. Whether you're growing plants for food or to enjoy their beauty, gardening can make your life better. Seize the moment!

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