Calculate rainwater collection potential from your roof. Estimate tank size and water savings.
Calculate how much rainwater you can collect from your roof.
The Rainwater Harvesting Calculator helps you determine the water collection potential of your roof and size appropriate storage tanks. Harvesting rainwater is an effective way to reduce water bills, conserve municipal resources, and provide irrigation during dry periods.
According to the American Rainwater Catchment Systems Association, a 1,000 square foot roof in an area with 30 inches of annual rainfall can collect approximately 18,700 gallons per year. This free water can offset 30-50% of outdoor water use and reduce stormwater runoff, benefiting local watersheds.
Collection efficiency is typically 75-85% due to first flush diversion, evaporation, and splash loss.
| Use | % of Home Use | Treatment Needed |
|---|---|---|
| Garden/Lawn | 30-50% | Screen only |
| Toilet Flushing | 20-30% | Sediment filter |
| Laundry | 15-20% | Fine filtration |
| Potable | 5-10% | Full treatment + UV |
Related tools: Pool Water Calculator, Annual Energy Cost Calculator
Collection = Roof Area x Rainfall x Collection Efficiency. 1 inch of rain on 1,000 sq ft = 623 gallons. Typical efficiency is 75-90%.
Size depends on rainfall patterns and usage. For a 1,000 sq ft roof with 30 inches annual rain, you can collect 14,000 gallons. A 500-1,000 gallon tank covers most garden needs.
Raw rainwater is not potable without treatment. It's best for irrigation, toilets, and laundry. Proper filtration and UV treatment can make it drinkable but requires testing.