Estimate how many gallons of rainwater you can collect from a roof, shed, greenhouse, garage, or garden structure. Calculate rain barrel storage, tank overflow, collection efficiency, garden watering coverage, monthly collection, and estimated water savings.
Roof area mode:
The calculator uses your known catchment area and rainfall amount to estimate collected rainwater.
Dimensions mode:
The calculator multiplies roof length by roof width to estimate catchment area, then calculates rainwater collection.
Storage mode:
The calculator compares collected water with rain barrel or tank capacity to estimate storage filled and overflow.
Garden use mode:
The calculator compares collected rainwater with garden water needs to estimate how many watering sessions or weeks it can cover.
A rainwater collection calculator helps estimate how much water can be captured from a roof or structure and whether your rain barrels or tanks are large enough.
It can help size rain barrels, estimate overflow, plan garden watering, compare monthly collection, reduce outdoor water use, and estimate water savings.
Your result shows collected gallons from one rainfall event, monthly collection estimate, total storage capacity, storage filled, overflow gallons, garden watering coverage, water saved, and estimated cost savings.
Multiply the roof catchment area by rainfall inches and 0.623 gallons. Then adjust for collection efficiency and first flush losses.
One inch of rain over one square foot produces about 0.623 gallons before collection losses.
Divide your expected collected gallons by the size of each barrel. Round up if you want enough storage for the full rain event.
Not all rain falling on a roof reaches the tank. Some water is lost to roof texture, gutter splash, leaks, debris filters, first flush diverters, and overflow.
Rainwater is commonly used for outdoor garden watering, but water quality can vary by roof material, debris, animals, storage conditions, and local rules. Do not assume roof-collected water is safe for drinking.