Rainwater Harvesting Calculator

The calculation multiplies your roof's collection area (length × width) by the rainfall depth, then applies a runoff coefficient for your surface type. The formula is: Volume (gallons) = Area (sq ft) × Rainfall (in) × 0.623 × Runoff Coefficient. The 0.623 factor converts cubic feet to gallons accounting for unit conversion.

The runoff coefficient represents the fraction of rainfall that actually flows off your roof into the collection system. Not all rain is captured — some evaporates or soaks into roofing materials. Metal roofs have coefficients near 0.95, while green roofs may be as low as 0.60. Choosing the right coefficient gives you a more accurate estimate.

For a simple rectangular roof, multiply its length by its width. Only include the portion of the roof that drains toward your collection downspout. For complex roof shapes, break the area into rectangles and add them together. The horizontal footprint area (not the slope area) is what matters for rainfall collection.

A 1,200 sq ft roof receiving 30 inches of annual rainfall can theoretically collect over 13,000 gallons per year with an asphalt shingle coefficient of 0.85. Results vary widely by roof size, local climate, and surface type. Use the calculator with your annual average rainfall figure to get a personalized estimate.

Tank sizing depends on how frequently it rains in your area and how much water you plan to use between rain events. For most residential uses like garden irrigation, tanks between 50 and 1,500 gallons are common. If you enter your tank capacity in this calculator, it will tell you whether your tank can hold the volume collected from a given rainfall event.

Harvested rainwater is commonly used for landscape irrigation, garden watering, ornamental pond and fountain filling, cooling tower make-up water, and — in appropriately filtered systems — toilet and urinal flushing. It is naturally soft and free of chlorine, making it excellent for plants. Potable use requires advanced filtration and treatment.

Rainwater harvesting regulations vary significantly by state and municipality. Many states like Texas and Arizona actively encourage it with incentives, while others have historically restricted it due to water rights laws. Always check with your local authorities before installing a system. Generally, collecting rainwater from your own roof for non-potable use is permitted across most of the US.

This calculator provides a solid theoretical estimate based on your roof dimensions, rainfall input, and surface runoff coefficient. Real-world collection may vary due to gutter efficiency, first-flush diverter losses, evaporation, and roof pitch. Use it as a planning guide to right-size your system, then consult a rainwater harvesting professional for a detailed site assessment.