About the rainwater collection calculator
Even a modest roof sheds a surprising amount of water. The calculation is simple: the roof’s footprint multiplied by the rainfall depth. Real systems collect less than that maximum because of gutter overflow, splash, and first-flush diversion, so you can set your own efficiency.
How to use this calculator
- Measure the roof footprint that drains to your downspout (length × width from above).
- Enter rainfall for one storm, a month, or a year.
- Set collection efficiency (100% is the theoretical maximum).
- Optionally enter barrel size to see how many barrels you’d fill.
Worked examples
- 1,000 sq ft roof section, 1 in of rain, 100%
- 1,000 × 1 × 0.623 = 623 gallons
- 55 gal barrels: 623 ÷ 55 = 11.3
About 623 gallons — far more than one barrel holds.
Measurement guidance
Which roof area?
Only the part of the roof that drains to your collection downspout counts. Check where each gutter section drains.
Local rules
Rainwater harvesting is encouraged in many places and restricted in some. Check local regulations, especially for large tanks.
Frequently asked questions
How much water comes off a roof in an inch of rain?
About 0.623 gallons per square foot of roof footprint, or 623 gallons per 1,000 sq ft.
What efficiency should I use?
100% gives the maximum possible. Lower it to reflect losses from overflowing gutters, splash, and first-flush diverters.
Is rainwater safe for gardens?
It’s widely used for watering ornamental plants and lawns. For edible crops, many guides recommend watering the soil rather than the leaves.