Roof Area (sq ft)


Inches of Rainfall


X 0.62

18,600 Gallons A Year

Rainwater Roof Calculator

How it’s calculated:

{Roof Area in square feet} x {inches of rainfall for your location} x 0.62 = Amount Collected

The equation is made up of 3 numbers:

  1. The size of your catchment area. Most likely your roof.
  2. The average inches of rainfall for where you live
  3. The average amount of water you can collect from 1 inch of rain per square foot of your catchment area (0.62 gallons)

Figure out your information with the steps below.

1. Rainfall By Location

As I’m certain you have already guessed, the amount of rainwater you can collect is heavily reliant on how much it rains where you live. This is obviously different for certain locations. After all, Seattle will experience more precipitation than Phoenix. The best place to start is the average rainfall for your location, which you can find on this map

Use the map below to roughly estimate how many inches of rainwater you can expect on average:

Additionally, if you want to get more granular, you can always Google “average inches of rainfall in (your city)” and you should get more specific information.

It’s probably also important to note that the amount of rain that falls changes year to year and even season to season. Sometimes you’ll experience a drought, other times you’ll experience heavy rain flow. Ultimately, this is impossible to guess but the goal of this calculator is to give you a baseline to understand your potential.

2. Calculating Your Roof Area

While calculating the square footage of your roof will become a complex task, starting with an educated “guesstimate” will be more than enough to understand the potential of your roof.

The easy way: use your home square footage

The most important key here is that the slope of your roof does not matter. While the slope can affect how many squares of shingles you might need to buy, your ‘catchment area’ is essentially the amount of ground covered.

So to get a really easy estimate, use the total square footage of your home to figure out what your max potential will be for collecting rainwater. This is also really easy to get just by finding your house on Zillow.

The hard way: breaking your roof into sections

Alright, now it’s time to get out paper, a pencil, and your tape measure. We can no longer rely on Zillow to give us a quick and easy answer.

It’s likely that you won’t be able to capture all of the rain that falls on your roof through one downspout. After all, the rain will drain in multiple directions so you’ll need to identify which sections of your roof you’re planning on collecting rainwater from. Consider which downspouts you are going to hook up to your system and measure the roof section that leads to that gutter.

Even still, a rough estimate of the size of the roof sections will likely be enough. We don’t experience the exact same amount of rain every year and we may lose some catchable rain for one reason or another so don’t worry about getting the exact square feet.

Read The Guide To Rainwater Harvesting

Check out our comprehensive guide to rainwater harvesting and its many benefits

Rainwater Calculator

*0.62 Gallons

100,000 Gallons Per Year