I was having a conversation about rainwater harvesting the other day. Specifically, we were discussing how to size rainwater collection systems to use rainwater for irrigation and animal watering.

I wanted to provide a resource on how to actually calculate the amount of rainwater you could actually collect off of a rooftop. It’s a pretty simple calculation.

First, you need to figure out big your roof will be. Now don’t worry, you don’t have to be exact! And don’t just look at your roof and try to figure out the angles, etc either. None of it is relevant!

The only thing matters is the square footage that the roof covers. And I mean literally that, covers. So if you have a 24×30 cabin, that roof will cover about 720 square feet.

**Rudy’s Note**: I’m serious. Forget about the angles and gables and all that. Rain doesn’t care. it’s basically a two dimensional problem here.

The next thing you need to find out is how much rainwater falls in your area. Now sometimes this can be tough since most places report overall precipitation and not just rain. You’ll need to figure this part out on your own.

I like using the site ‘HomeFacts‘ since their climate data has monthly precipitation numbers. You’ll probably know which months are rain vs snow, so this makes life easier!

Using Seattle as a guide, we find that our average annual precipitation is 38.25 inches. Since it doesn’t snow much in Seattle, we’ll pretend it’s all rain.

**Rudy’s Note**: If you DO want to include snowfall, it may be useful to know that on average 10 inches of snow is equivalent to one inch of rainfall. Slushy snow is less, light and fluffy powder is of course more than ten to one.

ANYHOW. Back to our rainwater collection system calculations!

Now we have all of the key information, so we pull out our handy calculator, and do the following:

Square Footage * Avg Rainfall (inches) / 12 = Rainfall in Cubic Feet

Now, we don’t really want just Cubic Feet, so we multiply the result by 7.48 to get rainfall in gallons.

For our example, it looks like this

720 sq ft * 38.25 inches / 12 * 7.48 = 17,166.6 gallons

So now we know that our little cabin in Seattle could potentially collect over seventeen thousand gallons of rainfall for us each year.

**Rudy’s Note**: Since this is all averages, and I’m lazy, instead of dividing by twelve and multiplying by 7.48, I just multiply by 0.623 (which is the result of 7.48 / 12) … it’s not going to give you the same result, but close enough:

*720 * 38.25 * 0.623 = 17,157 … Definitely close enough for my purposes!*

Anyhow, for what it’s worth, you now know how to calculate runoff for your new rainwater collection system! Or at least the rainwater harvesting you PLAN to do…

Very interesting! Just did mine:

950 sq ft roof * 37.96 inches avg rainfall = 36,062 / 12 inches = 3,005.1666 rainfall in cubic ft * 7.48 = 22,478.646 rainfall in gallons per year. Did I do this right? If so, that’s a lot of gallons!.

Sounds about right … pretty wacky, eh?

You should all include the usage amounts of a toilet, washer, etc for calculating storage amounts.

We always calculate you get 600 gallons for every inch of rain for every 1000 square feet. This underestimates your total by a little but still works. If you live in a city and have a water meter connected to your house it’s easy to calculate how much water you use. Just read the meter. On the other hand, when living in the country and using a well it’s a harder task. I found this site which will help calculate water use. http://www.csgnetwork.com/waterusagecalc.html