Rainwater Harvesting Tank Size Calculator

A rainwater tank has to answer two questions: can the roof collect enough to matter, and how long should stored water last between rains. Annual collection checks the roof; storage period sets the tank.

Rainwater Harvesting Tank Size Calculator uses catchment area, rainfall and household demand to estimate both the annual harvest and the recommended storage capacity.

• Catchment: Roof area with runoff coefficient.
• Rainfall: Annual amount for the region.
• Demand: Daily water use to be covered.
• Storage period: Days of autonomy between rains.
• Annual harvest: Roof potential per year.
• Tank size: Recommended working capacity.
• Export: Text, CSV or JSON report.

1. Enter the catchment area (roof footprint).
2. Set rainfall and runoff coefficient.
3. Add daily demand and storage days.
4. Read annual harvest and tank size.
5. Export the result if needed.

Example 1. A 1,000 sq ft metal roof in a 30 in/year region collects roughly 16,800 gallons annually - enough to buffer several hundred gallons a day for gardens.

Example 2. The same roof on gravel collects less than 11,000 gallons, and a tank sized to 30 days of demand must be checked against the roof ability to refill it.

• Two answers: Roof potential and tank size together.
• Roof aware: Runoff coefficient per surface type.
• Demand driven: Sizes to your actual use.
• Planning ready: Approximate capacity for ordering.
• Free and private: No uploads, no logging.