Calculate Your Rainwater Harvesting Potential Using This Simple Method

I see a lot of videos that claim they can save huge volumes of water. The most recent one was a guy who put an upturned lid on a barrel, collected water in it and piped that water to his tomatoes. But he clearly didn’t know how you can Calculate Your Rainwater Harvesting Potential

Are There Plant nutrients in Rainwater? Calculate Your Rainwater Harvesting Potential
Are There Plant Nutrients in Rainwater?

In theory his idea is a great idea. But let’s examine it a bit closer. How much water will a single barrel catch? Leave a bucket out in the rain and see if you get much. The reality is it catches quite a small amount of rain. It’s certainly not going to be enough to water a bed of tomatoes for the season. 

If the barrel is set up to catch the rain falling on a bigger area, say a roof, it’s a great idea. But in this case they guy was trying to irrigate a number of tomatoes from the water caught in one barrel WITHOUT it being attached to a roof or similar. 

Lets Look At How to Calculate Your Rainwater Harvesting Potential .. I’m getting to that, but first some proof.

Here’s the Mathematical Proof

In my area of Devon it’s not as wet as many think. We get around 30 inches of rain a year. Other parts of the country get more or less than this. An inch of rain is about 22,602 gallons per acre. So 30 inches is a nice easy figure to multiple by! 30×22,000 = 660,000 gallons an acre each year. It’s a lot of water. 

But my water barrel is only 2 ft across and that gives it an area of 3.14 square feet. That’s a very small area to catch water from. Especially when you realise that there are 43,560 square feet in an acre. 

The reality is that an inch of rain falling on a square foot is about half a gallon. So my small barrel with a surface area of just 3.14 square feet will catch around one and a half gallons of water.

That less than my watering can holds!

It’s not going to water many tomatoes in my greenhouse.

The Rain Water Harvesting Secret

OK, maybe it isnt a true secret because any reasonably intelligent person ought to realise that if you want to catch more water you need to harvest from a bigger area. A large roof will provide more water than a small one.

It’s not rocket science and I do wonder why people make videos about catching water in the lid of a barrel! People are strange.

Rain Water Harvesting Calculations aka How To Calculate Your Rainwater Harvesting Potential

Though an inch of rain is very slightly more than half a gallon per square foot let’s keep it simple and call it half a gallon.
It’s that simple, an inch of rain is roughly half an imperial gallon per square foot.

So to calculate how much your can harvest each year …… multiple the length by the width of your roof (or other catchment area) in ft.

Then multiply that figure by 0.5

Then multiply by the number of inches of rain you get each year. If you don’t know your local rainfall figure then assume 30 (it’s what I get and is an easy round figure).

Calculate Your Rainwater Harvesting Potential Example

Let’s start with very small roof space measuring 1 ft by 1 ft and one inch of rain


Multiply by 0.5 = 0.5

Multiply by 30 inches of rain per year


So 15 gallons per square foot per year.

Now let’s look at a real roof … one that’s a bit bigger.

Its 30 by 20 ft/

30×20 = 600 sq ft

600×0.5=300 gallon

300 by 30 inches of rain per year = 9000 gallons

This is a rough figure but accurate within a few gallons.

It is the total amount of rain falling on that roof area in a year. It assumes you can store all the water that falls on it. But of course that is rarely true because in winter your storage might be full for weeks and a lot of rainwater will not be stored.

If you can double your storage capacity you will possibly also double your total water harvested. But there is an upper limit which is the amount of rain that falls each year.

Financial Savings From Rainwater Harvesting

Where I live we are charged £1.9338 per cubic metre for water PLUS £3.2871 per cubic metre for foul water drainage. So saving rainwater is a valuable way in which to save money once you’ve paid for storage. And sometimes storage can be obtained via subsidised barrels or even free on Freecycle!

Rainwater also contains low level nutrients …. every little helps!

Enjoy your rainwater harvesting.

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