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How much rainwater can be captured from a roof?

How much rainwater can be captured from a roof?

A common question we often get is “how much rainwater can be captured from a roof”. This is an important question if you are planning to use the rainwater for whole house use (drinking) or irrigation. You have to know if it’s worth the expense of installing a raincatchment system and if there will be enough water available before you begin planning it out.

Here’s a quick rule of thumb on how much rainwater can be captured:

~625 gallons of rainwater can be captured per each inch of rain on a 1,000 square foot surface.

This means that a 2,000 square foot roof will yield ~1,250 gallons of rainwater for every inch of rain. Here in San Antonio, TX we average 32 inches of rain per year, so 1,250 x 32 = 40,000 gallons per year catchment potential.

Thats a lot of water!  So what can you do with it?  Is that enough to supply a household?

The short answer is “YES”.  Most people use about 50 gallons per day. Two people go through about 100 gallons per day/ 3,000 gallons per month. However, the average person on a rainwater catchment system will usually be more careful with their water usage and might use less than average.

Want to calculate your catchment surface?

  1. You don’t need to now how many square feet of roof “surface” you have, only the “projected” surface or the “bird’s eye view” of your roof. Since rain falls straight down (for the most part) it doesn’t matter if your roof is flat or has a steep pitch. If you don’t know your square footage multiply the width x length of your home to get the your square footage.
  2. Know the average rainfall in your area.
  3. Multiply surface area (sq ft) by 625 and then by yearly average rain. For example, if your catchment surface is 3,250 sq ft, …..multiply 3.25 x 625 x 32 = 65,000 gallons catchment potential per year.

Now, this doesn’t mean you need to have a 65,000 gallon tank. You get “free refills” so your tank will be smaller. It should be sized so that you occasionally overflow the tank but never quite drain it all the way. If you don’t ever overflow your tank it means either the tank is oversized or you’re using way more water than was figured when your system was designed.

There are thousands of rainwater catchment systems in operation today throughout Texas. There’s no doubt rain can supply the water we need to survive and thrive. People have been doing this since the beginning of time, we’ve just added modern technology to provide secure and safe water storage and clean filtration and purification of the rainwater.

We can give you estimates for the size of tank you need and the amount of water you can catch. Just give us a call!


Born in Texas, Ben was raised by missionary parents while traveling throughout Mexico, where he saw his first example of a simple yet functioning rainwater catchment system attached to a small sheet metal home. He went on to serve 5 years in the Navy before returning home to Texas, where he settled in San Antonio.