Rainwater Cistern Sizing Calculator
Balancing supply and demand
Sizing a rainwater cistern requires reconciling two numbers: how much water your roof can capture during an average month and how much your household consumes between storms. The calculator applies simple volumetric relationships so you can select tanks that prevent shortages without overspending on unused capacity. Because rainfall arrives unevenly, the math focuses on average conditions and autonomy buffers; you can layer local weather patterns on top of the baseline results for precision planning.
Monthly harvest volume combines roof area , rainfall depth , and efficiency :
Autonomy volume multiplies daily demand by the number of dry days : . Dividing by an average month (30.4375 days) yields typical daily harvest and highlights whether you expect a surplus or deficit.
Scenario comparison table
The table below explores how changing rainfall or demand shifts the recommended cistern size for a 100 m² roof. Use it as a quick reference when presenting design options to clients or community groups.
| Rainfall (mm/month) | Daily demand (L) | Harvest (L/month) | Autonomy 30 days (L) | Average daily balance (L) |
|---|---|---|---|---|
| 60 | 150 | 5,100 | 4,500 | 67 surplus |
| 80 | 200 | 6,800 | 6,000 | 23 surplus |
| 40 | 250 | 3,400 | 7,500 | −138 shortfall |
When harvest exceeds demand, consider smaller tanks with overflow routing or dedicating surplus to irrigation. In deficit scenarios, increase roof catchment, reduce consumption, or add backup supply to maintain service.
Integrating the cistern into a broader plan
Combine these sizing results with the Rainwater Harvesting Calculator to understand annual availability, then consult the Rain Barrel Sizing Calculator for smaller branch systems or the Rainwater Harvesting Payback Calculator to analyze costs. Document assumptions such as efficiency and autonomy so you can revisit them after monitoring real-world performance.
Regular maintenance—clearing gutters, inspecting inlet screens, and sanitizing storage—keeps water quality high and efficiency near your targets. With thoughtful planning, the cistern becomes a resilient buffer that supports gardens, toilet flushing, or emergency reserves during drought conditions.