How to Use the Rainwater Tank Savings Calculator
A rainwater collection tank catches runoff from your roof gutters and stores it for non-potable uses like garden irrigation, car washing, or toilet flushing. A larger roof area and higher local rainfall both increase how much you can collect — and how much you can save on your water bill. This calculator takes your roof area, annual rainfall, and collection efficiency to estimate your annual harvest and savings.
The math starts with the standard rainwater harvesting conversion: 1 square foot of roof under 1 inch of rain yields about 0.623 gallons. In practice, first-flush diverters (which filter out the initial dirty runoff), evaporation, and pipe losses reduce the actual amount collected, so we apply a collection efficiency of 70-85% to get a realistic estimate.
Annual rainfall varies widely across the US, from under 10 inches in parts of the Southwest to over 60 inches in the Pacific Northwest and Southeast, so check your local average before sizing a system. Tanks are typically sized to hold roughly 5-10% of the annual harvest — large enough to avoid overflowing after a single storm, but not so large that it sits mostly empty. Savings are based on your local water rate and reflect the reduction in metered water use.
Frequently Asked Questions
Multiply your roof area (sq ft) by rainfall (inches) by 0.623 — the standard conversion for gallons collected per square foot per inch of rain — to get the theoretical harvest. We then apply a collection efficiency (typically 70-85%) to account for first-flush diversion, evaporation, and pipe losses.
The ideal size holds enough to avoid running dry during hot, dry stretches without overflowing after a single heavy storm. Most residential systems use roughly 5-10% of the estimated annual harvest, commonly a 50-275 gallon tank.