Rainwater Harvesting Tank Sizer

Plan cistern capacity with a quick balance of inflow and demand. Enter catchment size, rainfall, runoff efficiency, and planned usage to see how many litres you should store for the target month.

Total roof or surface area feeding the system.
Rainfall depth for the planning month.
Efficiency of the surface in directing water to storage.
Water you intend to use during the month.
Optional. Defaults to 1 mm removed for debris flushing.
Optional. Defaults to 0.65 to retain part of monthly surplus as buffer.

Examples

  • 120 m² roof, 80 mm rainfall, coefficient 0.85, demand 1,800 L, defaults for losses ⇒ recommended tank 5,867.70 L
  • 360 m² roofs, 100 mm rainfall, coefficient 0.85, demand 12,000 L, defaults ⇒ recommended tank 23,891.10 L

FAQ

How should I pick the storage fraction?

Set it higher (0.7–0.85) when you expect longer dry spells or want more resilience, and lower when frequent storms refill the tank.

Does this include evaporation losses?

No. Add evaporation or leakage as extra demand if your storage is exposed or unsealed.

Can I adapt this for annual planning?

Yes—enter annual totals divided into representative months, then average the recommendations or choose the peak requirement.

Additional Information

  • Monthly harvest equals catchment area × effective rainfall × runoff coefficient, yielding litres directly.
  • First-flush removal subtracts a thin layer of rainfall to divert debris and improve stored water quality.
  • Storage fraction shapes how much surplus or deficit is buffered so the tank size covers dry spells without oversizing.

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