Before you buy
Practical measuring tips
- Use horizontal roof footprint, not sloped surface area.
- Efficiency accounts for first flush, splash, leakage, and other losses.
- Check local rules and use safe materials for the intended water use.
Water
Rainwater Collection Calculator — Gallons
Estimate harvestable gallons from roof area and rainfall. Your entries stay in your browser and are not sent anywhere.
Planning estimate
Your results
Formula
What the result includes
Roof area × rainfall inches × 0.623 gallons per ft²-inch × collection efficiency.
Whole-item results such as bags, rolls, blocks, and pallets are rounded up. Decimal volume or weight results remain estimates so you can compare bulk options.
Optional supplies
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Use your estimate to compare package coverage, delivery terms, and product instructions. This retailer search is not an affiliate link until a partner ID is configured.
Common questions
Rainwater Collection Calculator FAQ
Where does 0.623 come from?
One inch of water over one square foot is approximately 0.623 US gallons.
Does this predict annual supply?
No. It estimates one rainfall amount. Storage, dry periods, overflow, and demand also matter.