Free rainwater harvesting calculator. Estimate how much rainwater you can collect from your roof using catchment area, rainfall, and efficiency.
Diverts the first, dirtiest runoff of each storm — cleaner water, about 10% less volume.
See how far your harvest goes for a given use and household size.
Add a tank size and daily use to see fill level and days of supply.
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Rainwater harvesting is a sustainable practice that captures and stores rainwater for later use. Our calculator helps you estimate how much water you can collect from your roof based on your local rainfall, roof size, and collection system efficiency.
Every inch of rain on 1 ft² of roof yields about 0.623 gallons (144 in³ ÷ 231 in³ per gallon); in metric, 1 mm on 1 m² yields exactly 1 liter. Multiply catchment area by rainfall and the roof's runoff efficiency to estimate harvestable volume. This is the standard method in the Texas Manual on Rainwater Harvesting (TWDB) and ARCSA guidance.
Collection Formula
Plants prefer soft rainwater over chlorinated tap water.
Provide clean water for chickens, goats, and other animals.
Have water available during power outages or supply disruptions.
Soft rainwater is excellent for outdoor cleaning tasks.
Collected rainwater can be used for irrigation, washing, and with proper treatment, even drinking.
Store water during wet periods for use during dry spells or water restrictions.
Reduce stormwater runoff and decrease demand on municipal water systems.
Metal roofs can achieve 90-95% efficiency, while asphalt shingles are typically 80-85%. Green roofs may only yield 50% as they absorb significant rainfall.
Size depends on your usage and local rainfall patterns. A good rule is to store at least one month's worth of expected collection or usage, whichever is greater.
With proper filtration, UV treatment, and storage, rainwater can be potable. Check local regulations and consult water quality experts before using for drinking.
A first flush diverter discards the initial rainfall that washes debris, bird droppings, and dust off your roof. This improves water quality but reduces overall collection by 5-10%.