Estimate rain barrel size, rainwater collected, number of barrels needed, overflow, storage filled, garden watering coverage, and estimated water savings from roof runoff, shed roofs, greenhouse roofs, garages, and garden structures.
Rain barrel storage mode:
The calculator compares collected rainwater with your total rain barrel capacity and current stored water.
Barrels needed mode:
The calculator estimates how many rain barrels are needed for a target storage amount or collected rainwater amount.
Roof collection mode:
The calculator estimates gallons collected from roof area, rainfall amount, collection efficiency, and first flush loss.
Garden coverage mode:
The calculator compares stored rainwater with garden water needs to estimate watering sessions and weeks covered.
A rain barrel calculator helps estimate how much water a barrel system can store and whether the setup is large enough for your roof and garden.
It can help size rain barrels, avoid overflow problems, estimate watering coverage, plan storage upgrades, compare roof catchment areas, and estimate outdoor water savings.
Your result shows estimated collected gallons, total rain barrel capacity, barrels needed, storage filled, overflow gallons, garden watering sessions covered, weeks of garden water, and estimated water savings.
Estimate the gallons collected from your roof during a rain event, then divide by the size of each rain barrel. Round up to the next whole barrel.
It depends on roof area, rainfall, collection efficiency, and storage size. A larger roof can fill a small barrel quickly during a moderate rain.
One inch of rain over one square foot produces about 0.623 gallons before collection losses.
Overflow happens when collected water exceeds available storage. Add more barrels, increase tank size, or route overflow safely away from problem areas.
Rain barrel water is commonly used for outdoor garden watering, but quality can vary by roof material, debris, animals, storage conditions, and local rules. Do not assume roof-collected water is safe for drinking.