💧 Rainwater Harvesting Calculator
Calculate how much rainwater you can collect from your roof and how much you could save on water bills.
Roof & Rainfall Details
m²
mm
£/$/€
per m³
Runoff Coefficient by Material
| Material | Coefficient | Efficiency |
|---|
Common Uses for Harvested Rainwater
Garden Irrigation
~200L/week (avg garden)
Toilet Flushing
~50L/person/day
Car Washing
~200–400L per wash
Laundry
~60L per wash (check softness)
How Rainwater Harvesting Works
Rainwater is collected from your roof, filtered through a first-flush diverter, and stored in a tank. The usable volume depends on your roof area, its material (which affects how much rain runs off versus soaks in or evaporates), and rainfall in your area.
Formula: Monthly collection (L) = Roof Area (m²) × Rainfall (mm) × Runoff Coefficient × 0.85 (filter/pipe losses)
Tank size recommendation is based on 2 months of average collection — enough to buffer through a dry spell without overflow during wet months.