🪣 Rainwater Harvesting & Tank Simulator

Everyone sizes a water butt by guessing. This runs a day-by-day simulation of a whole year — rain falling on your roof, water drawn off, the tank filling, overflowing and running dry — then sweeps every tank size from 100 to 20,000 litres to show you exactly where extra capacity stops buying you anything.

Rainfall pattern

Roof plan area (m²)

Roof surface

Tank size (litres)

Everyday demand (litres/day)

Extra summer garden use (litres/day in July)

Average wet days per month

First-flush diverted per wet day (litres)

Mains water price (£ per m³, supply + sewerage)

System cost installed (£)

Monthly rainfall, millimetres — edit any month

Figures are approximate long-run monthly normals, rounded — good enough for sizing a tank, not a climatology dataset. Replace them with your own gauge readings if you have them.