Rainwater Harvesting Tank Size Calculator
Designing a reliable rainwater harvesting system starts with understanding how much runoff your roof can produce and how many dry days you need to bridge. This calculator estimates each step by combining roof catchment area, local rainfall, collection efficiency, daily demand, and the desired number of storage days. The multiplier 0.623 converts one inch of rain on one square foot of roof into gallons, which is standard in hydrology sizing.
Enter your site-specific values below to see the annual harvestable volume, average daily capture, and a realistic tank recommendation that accounts for both your water demand and the amount of rainfall you can actually store.
How the calculation works
The calculator multiplies your roof area by total seasonal rainfall and by 0.623 gallons per square foot-inch, then applies the efficiency factor to account for losses from first-flush diverters, wind, or gutter leaks. Dividing the annual volume by 365 yields the average daily capture potential. The tool compares your desired storage (daily demand × autonomy days) with what the rainfall can realistically support. The lower of the two numbers becomes the recommended tank capacity so that you do not oversize a tank beyond what you can fill, yet still meet as many dry days as possible.
Example sizing
For a home with a 1,200 sq ft roof in an area receiving 32 inches of rain annually, assuming 85% efficiency, a household demand of 35 gallons per day, and a goal of 21 storage days:
- Annual harvestable water = 1,200 × 32 × 0.623 × 0.85 ≈ 20,377 gallons
- Average daily capture ≈ 55.8 gallons per day
- Desired storage = 35 × 21 = 735 gallons
- Recommended tank ≈ 735 gallons because rainfall can comfortably replenish that volume
If local rainfall were much lower or demand much higher, the calculator would flag that the storage target exceeds what your roof can supply, prompting you to add catchment area, improve efficiency, or reduce usage.