Rainwater Harvesting Calculator
Work out how much rainwater a roof collects. Enter the footprint, rainfall and roof surface for one storm or a year, and see the barrels or tank it fills.
How to Use
- Choose one storm or a whole year.
- Enter the roof footprint, the area the roof covers seen from above, not the sloped surface.
- Enter the rainfall: the storm depth, or your annual average from a weather service.
- Choose the roof surface; smoother roofs shed more of the rain.
- Enter your tank size to see the overflow from a storm or how many times a year it refills.
Worked Example
A house with a 1,500 ft² roof footprint, asphalt shingles, and a 1 in storm. One inch of rain on one square foot is 144 in³, which is 144 ÷ 231 = 0.6234 US gallons. So 1,500 × 1 × 0.6234 = 935 gal falls on the roof, and with a runoff coefficient of 0.90, 842 gallons reach the gutters. That fills 842 ÷ 55 = 15.3, so 16 rain barrels, and weighs 842 × 8.34 = 7,020 lb, 3½ tons. A 500 gal tank would overflow by 342 gal.
Over a year with 38 in of rain and a metal roof (0.95): 1,500 × 38 × 0.6234 × 0.95 = 33,756 gal, an average of 2,813 a month. A 1,500 gal tank would fill and empty about 22½ times a year if the water is used between storms.
The common mistake: measuring the sloped roof. A 6/12 roof surface is 12% larger than its footprint, but it catches no more rain, because rain falls vertically. Using the surface area overstates the harvest by 12% at 6/12 and 41% at 12/12.
Show Work
Formulas
Cisterns, Old and New
People have stored rain from roofs and courtyards for thousands of years. The Basilica Cistern under Istanbul, built in the 6th century, holds about 80,000 m³, and plastered cisterns under houses were standard in dry places from the Middle East to Bermuda, where building rules still require homes to collect their own water from stepped white limestone roofs.
Today rainwater is mostly used for gardens, toilet flushing and laundry, which together are often half of a household’s water. A first-flush diverter sends the first few gallons of each storm, which carry dust, pollen and bird droppings, away from the tank, and a screened, sealed tank keeps out light, leaves and mosquitoes.
About This Calculator
This calculator works out the rainwater a roof sheds in one storm or over a year from its footprint, the rainfall and the runoff coefficient of the surface, in US gallons or litres, with the number of 55 gal barrels (200 L water butts), the weight of the water, and how your tank copes. It draws the roof, the downpipe and the tank fill.
Everything runs in your browser; nothing is sent anywhere. Annual figures assume the tank is used between storms; a full tank collects nothing. Water is heavy, so set tanks on a level, compacted base, and check local rules before using rainwater indoors.
Related calculators: Gutter Calculator, French Drain Calculator, and Roofing Calculator.
Frequently Asked Questions
How much water comes off a roof per inch of rain?
About 0.62 gallons per square foot of roof footprint for every inch of rain, before losses. A 1,000 ft² roof gives 623 gallons per inch in theory, about 560 from shingles. In metric it is simply 1 litre per square metre per millimetre.
Do I use the roof area or the footprint?
The footprint, the area of the ground the roof covers. Rain falls vertically, so a steep roof catches no more than a flat one over the same ground. Measure the outside of the eaves, length times width.
What runoff coefficient should I use?
It is the share of the rain that reaches the gutter: about 0.95 for metal, 0.90 for asphalt shingles, 0.85 for tile, 0.70 for a gravel flat roof and 0.40 or less for a green roof. The rest is lost to wetting, evaporation, splash and the first-flush diverter.
How big a rainwater tank do I need?
Big enough for a typical storm, or for the dry spell you want to bridge. A 1,500 ft² shingle roof fills a 55 gal barrel with 0.07 in of rain, and gives 842 gal from a 1 in storm. Many homes use 1,000 to 3,000 gal tanks for garden use; whole-house supply needs much more and a demand calculation.
Is collecting rainwater legal?
In most places, yes, and many cities offer rebates for barrels. A few US states, such as Colorado, limit how much a household may store, and drinking-water use usually needs treatment and permits. Check your state or local rules.
How do I use the Rainwater Harvesting Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
One storm
1,500 ft² of shingles, 1 in of rain: 842 gal, 16 barrels, weighing 3½ tons.
A whole year
The same footprint with a metal roof and 38 in a year: 33,756 gal, about 2,800 a month.
Garden shed
120 ft² metal roof, ½ in of rain: 36 gal, under one barrel.
Metric
100 m² of tile and a 25 mm storm: 2,125 L, more than ten 200 L water butts.
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