How this calculator works
One inch of rain falling on a square foot of surface is 144 cubic inches of water, which is 0.623 US gallons. So the potential harvest is roof area × rainfall × 0.623. We multiply by an efficiency factor to allow for splash, evaporation and the first flush you divert to clean the roof.
Worked example: a 1,500 ft² roof in a 1 in storm at 85% efficiency collects 1,500 × 1 × 0.623 × 0.85 = 794 gallons, enough to fill about 14 barrels of 55 gallons.
Tips before you buy
- Rain barrels fill fast. A single inch of rain on an average roof is more than a dozen barrels, so fit an overflow.
- Use stored rainwater on gardens and for washing. Do not drink it unless you have a proper treatment system.
- Add a first-flush diverter to avoid washing roof dust, bird droppings and pollen into the tank.
- Check local rules. Some regions encourage rainwater harvesting with rebates, and others restrict it.
Tools & materials for this project
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Frequently asked questions
How much water does 1 inch of rain give on a roof?
About 623 gallons for each 1,000 ft² of roof, before losses. A 2,000 ft² roof gets 1,246 gallons from a 1 in rain.
How many rain barrels do I need?
It depends on how much you want to store. A 1,500 ft² roof can fill a dozen barrels in a single inch of rain, so most people use one to four and let the rest overflow into the garden.
Does the roof pitch matter?
No. Rain falls vertically, so the area that matters is the roof's footprint, not its sloped surface.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.