Rainwater & storage volume

Rainwater: enter the horizontal projection of the connected collection area, annual rainfall, and collection efficiency after the losses you assume. A sloping roof surface is not the horizontal catchment. 1 mm on 1 m² is 1 L; potential yield = area × rainfall × efficiency/100. No local weather data are fetched.

Annual demand assumes the same daily demand for 365 days. Yield minus demand is the annual surplus or deficit before timing, overflow and storage losses. A surplus does not guarantee year-round supply. Dry-period reserve = daily demand × days with no inflow; it assumes that this amount of usable water is present at the start.

Optional existing tank: enter usable water volume, not nominal size, excluding dead volume. Days without inflow = usable volume / daily demand; missing dry reserve = max(0, reserve − usable volume). Zero demand gives no finite days figure. No recommended cistern capacity is inferred: actual sizing needs rainfall and demand over time, starting fill, overflow, usable storage and desired reliability.

Hydraulic retention: people × per-person inflow = daily flow; hydraulic liquid volume = daily flow × chosen retention time. For example, 4 × 150 L/day × 2 days gives 1200 L, without an automatic 50% allowance. The example is not a septic-tank specification: sludge, scum, peak flow, treatment, tank geometry and local requirements are absent. Choose the inputs from your scenario; initial numbers are examples.

Calculate or press Enter; edits and mode changes remove old results. Reset restores example fields in the current mode. Zero rainfall, efficiency, demand or dry-period duration are allowed; people must be a positive integer and retention time positive. Input bounds are software limits. No inputs are stored.