This calculator estimates collectible rainwater volume from roof catchment area and annual rainfall, then applies collection efficiency and first-flush/system losses to estimate usable annual supply. It also estimates demand coverage and annual water-value offset based on your usage and local water rates.

Practical Design Tips

Use monthly rainfall and demand profiles for final system sizing. Annual averages are useful for early planning, but storage reliability and overflow behavior require seasonal analysis.

• Use local historical rainfall data rather than broad regional averages
• Model separate demands for irrigation and indoor non-potable use
• Include filtration and treatment plans in lifecycle cost assumptions
• Right-size storage to reduce overflow during peak rainfall months
• Verify code, permitting, and cross-connection compliance requirements
• Pair water strategy with energy-efficiency improvements for better resilience

Sources & References