Calculate how much rainwater you can collect from a roof, shed, greenhouse, barn, or other catchment area. Estimate gallons collected, rain barrels needed, tank storage, overflow, water savings, collection efficiency, and storage days.
Roof area collection:
Enter roof catchment area and rainfall depth to estimate the water available for collection.
Roof dimensions:
Enter roof length, width, and pitch factor to estimate roof catchment area before calculating collection volume.
Storage tank planning:
The calculator compares collected water to tank capacity and estimates overflow, fill percentage, and barrels needed.
Water demand coverage:
Enter daily water use to estimate how many days your collected rainwater can supply irrigation or outdoor water needs.
A rainwater collection calculator helps estimate how much water can be captured from a roof and how much storage is needed.
It can be useful for rain barrels, cisterns, garden irrigation, greenhouse watering, livestock water planning, stormwater reduction, drought preparation, and water conservation projects.
Your result shows potential rainwater, collected water after efficiency and first flush losses, tank storage, overflow, barrels needed, tank fill percentage, days of water supply, liters, cubic feet, cubic meters, and estimated water value.
Multiply roof area by rainfall depth. A quick estimate is roof square feet × rainfall inches × 0.623 gallons, then multiply by collection efficiency.
One inch of rain on 1,000 square feet produces about 623 gallons before collection losses.
Some water is lost through splash, leaks, gutter overflow, first flush diversion, evaporation, wetting the roof surface, and system inefficiency.
Divide the collected gallons by the size of each barrel, then round up to the next whole barrel or tank.