Rainwater Calculator
See how much water you can collect from your roof. Enter the roof area and rainfall to get gallons per storm and per year, how many rain barrels or what size tank you need, and how much rain it takes to fill one.
Add monthly rainfall for a month-by-month estimate
| Month | Rainfall | Collected |
|---|---|---|
| Total | – |
How to calculate rainwater collection
- Enter your roof’s footprint: the length and width of the area that drains to the gutter you will use, measured on the ground, not along the slope.
- Choose the roof surface, then enter the rainfall for one storm and your area’s annual average.
- Read how much water you can catch, how many barrels a storm would fill, and how little rain it takes to fill one barrel.
The rainwater formula
Every inch of rain on one square foot of roof produces 0.623 US gallons of water (one millimeter on one square meter gives one liter). Multiply by the roof area and the rainfall, then by a runoff coefficient for the surface, because some water splashes off, soaks in or evaporates. Metal roofs shed about 95%, asphalt shingles about 90%, and gravel or green roofs much less. Gutters overflowing in downpours lose a little more, which the efficiency setting covers.
Use the roof’s footprint, the area it covers on the ground, not the sloped surface area. A steep roof catches no more rain than a flat one with the same footprint.
It adds up fast
A 1,200 sq ft roof collects around 670 gallons from a single inch of rain, enough to fill a dozen 55-gallon rain barrels. Even a small shed roof fills a barrel in a light shower. Most homes can collect far more water than they can store, so size storage for how you will use the water, such as a week of garden watering, rather than for the biggest storm.
Tips
- A first-flush diverter sends the first gallon or so per 100 sq ft, which carries dust, pollen and bird droppings, away from the barrel.
- Screen inlets to keep out mosquitoes and leaves, and give each barrel an overflow pipe that leads away from your foundation.
- Untreated rainwater is great for gardens, lawns and washing cars, but is not safe to drink without proper filtration and disinfection.
- Most US states allow or encourage rain barrels; a few limit the amount or require permits for larger systems. Check local rules.
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Frequently asked questions
How much water does an inch of rain produce?
0.623 US gallons per square foot of roof, or about 623 gallons per 1,000 sq ft, before losses.
How many rain barrels do I need?
Divide the water from a typical storm by your barrel size. Many people start with one or two barrels per downspout and link more together as needed.
Where can I find my annual rainfall?
Your national weather service publishes long-term averages; in the US, NOAA climate normals list average monthly and annual rainfall for most towns.
Is it legal to collect rainwater?
In most places, yes, and some areas offer rebates. A few US states limit how much you can store or require a permit for large systems, so check before installing big tanks.
Is anything sent to a server?
No. Everything is calculated in your browser, and nothing you enter leaves your device.