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Rainwater Harvesting Calculator

Plan a simple rainwater balance for a roof catchment. Enter monthly rainfall, runoff efficiency, tank size and expected non-potable use to estimate collection and storage through the year.

Planning estimate only. The monthly model adds rain before subtracting demand and does not model daily timing, first-flush systems, leaks, evaporation or roof-specific runoff. Rainwater may need treatment and may not be safe to drink; check local rules and qualified water-safety guidance.

How much rainwater can a roof collect?

One millimeter of rain on one square meter produces one liter before runoff losses. Multiply each month’s rainfall by catchment area and the runoff percentage; this estimate then adds captured water to the tank, limits storage to capacity and subtracts that month’s demand.

Frequently asked questions

How much rainwater can a roof collect?

One millimeter of rain on one square meter produces one liter before runoff losses. Multiply each month’s rainfall by catchment area and the runoff percentage; this estimate then adds captured water to the tank, limits storage to capacity and subtracts that month’s demand.

Are my roof or water-use numbers uploaded?

No. The balance runs locally in your browser. Your entries are not sent to a server or saved by this tool.

How do I estimate roof runoff?

Use a runoff percentage appropriate to your roof and collection setup. This tool does not select that value because materials, gutters, first-flush diversion and maintenance vary.

Can I drink the collected rainwater?

This calculator does not test or treat water. Safety depends on the roof, storage, treatment and local requirements; do not assume harvested water is potable.