All these products are great, and you know you want to catch rain.

But how much do you need? And how much can you even catch?

Use this handy dandy calculator to find out, based on your exact roof and local rainfall. Read on, and prepare to become a rainwater calculating expert…

Step 1: Find your roof size

Enter your address below, zoom in and click on each corner of your roof until your entire roof is highlighted — you should see the total area in area in square meters and square feet.

Step 2: Play around with our calculator.

The second step is to download or open up the spreadsheet here.  Wahoo! It’s free!

*UPDATE: Enter your city below and I’ll send you a copy with your city rainfall already in it.

It is ridiculously simple and (hopefully) easy to use.  Here is a screenshot of what it looks like:


Then, just enter numbers in the labeled, highlighted cells:

  1. your roof size (in L x W or square feet, found in Step 1),
  2. number of people in your household,
  3. select your state (all 50 are available)
  4. select your nearest city (pick from 5 cities in each state)

…and it tells you:

  1. how many gallons per month and year you can potentially collect, and
  2. how many gallons per month and year you likely use, (default water use based on WI averages — you can adjust if you know what yours is)

Don’t be afraid of wiping out formulas or screwing it all up. It’s protected. And if you lose the file, just download a new one!

Now you’re ready to start designing your system.  Check out our DIY Rainwater Guide for more ideas, here is the very first edition / beta, aka Free for you!  Why?  because we like you, but also a fair warning: it’s not yet complete! (plans for some of the stuff in it are coming soon!)

DIY Rainwater Guide

Lonestar Geyser

We finally found the end of the rainbow. Pot of gold?  Nope, Better:  this Ultimate DIY Guide to Rainwater Harvesting… too bad some dude is standing right in front of it