Rainwater Collection Calculator
Estimate how much rainwater you can collect from a roof or catchment surface based on area and rainfall.
Calculate rainwater collection
Enter the catchment area and rainfall amount. This calculator estimates collected water in US gallons and litres.
623 gallons
Based on 1,000 sq ft of catchment area and 1 inch of rainfall.
How to use this calculator
Enter the footprint area of your roof or catchment surface in square feet, then a rainfall amount (one event, a storm total, or a monthly average). The calculator multiplies these by 0.623 gal/sq ft per inch, the standard hydrologic coefficient for impermeable surfaces, and converts to litres.
Tips for success
- A common estimate is 0.623 gallons per square foot per inch of rain.
- Real-world collection is usually somewhat less due to losses and inefficiency (10-15% loss is typical).
- Clean catchment surfaces and proper filtration matter.
- Stored rainwater can be useful for gardens and non-potable uses.
- Local laws may affect rainwater collection in some places.
Practical note
This is a valuable tool for preparedness, garden watering, and self-reliant water planning. Standard rain barrels hold 55 gal / ~208 L, use the yield estimate to size your storage.
FAQ
How much rain can I collect from 1,000 square feet of roof?
About 623 gallons per inch of rainfall, using the standard 0.623 gal per sq ft per inch estimate. Real yield is usually 85 to 90 percent of that after losses from splash, first-flush diverters, and evaporation, so closer to 530 to 560 gallons per inch. A single 1-inch rainstorm on a 1,000 sq ft roof fills about 10 standard 55-gallon barrels.
Is rainwater collection legal?
In most US states and Canadian provinces it's legal and even encouraged with rebates. A few western states (Colorado, Utah) have historical water-rights restrictions but most now allow residential collection up to certain limits. Check your local water authority or extension office before building a large cistern system, especially if you plan to use the water for anything beyond landscape irrigation.
Can I drink harvested rainwater?
Not without treatment. Rainwater picks up bacteria, roof debris, bird droppings, and airborne contaminants. For potable use you need first-flush diversion (discards the dirty first few gallons of each storm), sediment filtration, carbon filtration, and UV or chlorine disinfection. For garden, laundry, and flushing toilets, basic filtration is usually sufficient.
What roofing material gives the cleanest rainwater?
Smooth metal roofs (galvanized steel, aluminum, standing seam) give the cleanest yield with the least debris and chemical runoff. Concrete tile is next best. Asphalt shingles shed small amounts of grit and petroleum residue, so water off them should not be used for drinking without filtration. Wood shakes, chemically treated cedar, and lead-flashed roofs are not recommended for any collection.
How big a rain barrel or cistern do I need?
Size storage to capture about 1 inch of rainfall from your roof without overflowing. For a 1,000 sq ft roof that's about 550 to 600 gallons of storage, or ten 55-gallon barrels. For whole-property irrigation and preparedness, sizing for the largest typical rainstorm (1.5 to 2 inches) means 800 to 1,200 gallons. Above that, cisterns make more sense than barrels.