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Easily calculate how much rainwater you can collect with our rainwater catchment calculator. Learn the formula, steps, and common questions answered.
Rainwater is free. You only need the right setup to collect it. That’s why we built a rainwater catchment calculator. It shows you how much water your roof can harvest. With this tool, you’ll know exactly how much water you can store for later use. It’s quick, easy, and accurate.
The math is simple. The formula looks like this:
V = A × P × C × e
Where:
In plain words: multiply your roof size by rainfall. Then adjust for roof type and system loss.
Say your roof is 100 square meters. The rainfall is 50 millimeters. Your roof has a coefficient of 0.9, and your system runs at 95% efficiency.
V = 100 × 50 × 0.9 × 0.95
V = 4275 liters
That’s about 1129 gallons. Enough to fill over 20 standard rain barrels.
This is a common question. The quick answer: 1 inch of rain on 1 square foot gives 0.623 gallons.
So if you have a 1,000 square foot roof and 1 inch of rain, you’ll collect about 623 gallons.
In most places, yes. Some states or countries have rules on how much you can store. A few even offer rebates for setting up rainwater systems. Always check your local laws before you start.
One inch of rain on only 89 square feet of roof will fill a 55 gallon barrel. So even a small shed roof can supply a lot of water fast.
It’s that simple. You’ll also see a step-by-step breakdown so you can trust the math.
Rainwater harvesting isn’t rocket science. With the rainwater catchment calculator, you’ll know how much water you can collect after any storm. It helps you size your tanks, plan your garden irrigation, or even cut down your water bill. Give it a try and see how much free water you’re missing.
It can be, but you’ll need filters and treatment. Most people use it for gardening, flushing toilets, or washing cars.
Very accurate. It uses the same formula engineers and environmental experts use.
Yes. Metal roofs give a higher runoff than clay or asphalt. That’s why the calculator includes a runoff coefficient.
Yes. It works with square feet, inches, gallons, or square meters, millimeters, and liters.