Inputs and units
Enter the horizontal roof footprint that drains into the collection system in square feet, rainfall depth in inches and a collection-efficiency assumption. Use only the portion connected to the tank. The sloping surface area of a pitched roof is not the horizontal catchment area. The optional people field converts gallons to a simple one-gallon-per-person-day planning figure.
How the calculation works
Collected US gallons = roof footprint (ft²) × rain (in) × 0.6234 × collection efficiencyAn inch of rain spread over one square foot is about 0.6234 US gallons. Efficiency reduces that theoretical amount to allow for losses. The preset percentages are scenarios, not tested performance for your roof. First-flush diversion, gutter capacity, leaks and rainfall intensity can change what actually reaches storage.
Worked example
A 1,000 ft² catchment receiving one inch of rain has a theoretical yield of 623.4 gallons. At 85% collection efficiency the estimate is 529.89 gallons, displayed as 530. If the tank has only 200 gallons of free capacity when the event starts, it cannot retain all 530 gallons without additional storage or concurrent use.
Assumptions and limits
This is not a rainfall forecast or dry-season reliability model. The days-of-water output does not include all household demand and does not establish that runoff is safe to drink. Roof materials and contamination affect appropriate uses; screening, filtering or disinfection alone may not remove every hazard. Assess treatment and water quality for the intended use, and check local collection and plumbing requirements.
References and calculation details
Page prepared September 18, 2026. Examples describe this calculator’s implementation; reference links do not imply independent certification.