Rainwater Harvesting Calculator
Rainwater you can collect from a roof each year, and the tank size to fit.
About this tool
Enter the roof's footprint area, your annual rainfall and the roof material. You'll see the water you can collect per year and per month, how much one 25 mm (1") rain gives, a tank size by the 5% rule of thumb or for a number of dry days, and how much of your daily use it could cover.
How much rain can a roof collect?
The basic relationship is simple: 1 mm of rain falling on 1 mยฒ of roof is 1 litre of water, and 1 inch on 1 square foot is about 0.623 US gallons. Real roofs lose some water to splash, evaporation, wetting the surface and overflow, which the runoff coefficient allows for: about 0.95 for metal, 0.9 for tiles and slate, 0.85 for asphalt shingles, 0.7 for flat roofs and as low as 0.5 for green roofs. Filters and first-flush diverters discard roughly another 10%. A 100 mยฒ tiled roof in a place with 600 mm of rain a year can supply about 48,600 litres. A 1,000 sq ft roof with 30 inches of rain gives about 15,000 US gallons. Measure the footprint, the area the roof covers seen from above, not the sloping surface, and only count the parts that drain to your downpipes. For a simple system, UK guidance (BS 8515) suggests a tank of about 5% of the yearly yield or the yearly demand, whichever is smaller. Alternatively, size it to carry you through a typical dry spell at your daily use. Honest limits: rain arrives in bursts, so small tanks overflow in wet weeks and run dry in droughts. Harvested rainwater is fine for gardens, toilets and washing machines, but needs proper filtration and disinfection before drinking or cooking. Some areas have rules on collection or require backflow protection. The pond liner calculator, the water intake calculator and the pool volume calculator are related.
Frequently asked questions
How much water can I collect from my roof?
Roof footprint (mยฒ) ร rainfall (mm) ร runoff coefficient gives litres per year. 100 mยฒ ร 600 mm ร 0.9 is about 54,000 litres before filter losses.
How many gallons per inch of rain?
About 0.623 US gallons per square foot, which is 623 gallons per 1,000 sq ft of roof for every inch.
What size rainwater tank do I need?
A common rule of thumb is 5% of the yearly yield or yearly use, whichever is smaller. Alternatively, size it for the number of dry days you want to cover.
Is anything stored?
No. Calculations run in your browser and nothing is stored or sent.