Rainwater Harvesting Tank Calculator

Harvested rainwater is suitable for WC flushing (largest non-potable use in homes), washing machines, garden irrigation, car washing, and general cleaning. It should NOT be used for drinking, cooking, or bathing without specialist treatment and UV disinfection.

WC flushing, washing machines and garden use together make up roughly 30-50% of typical household water use (Waterwise / Energy Saving Trust), so rainwater can in principle cover that share. For a 4-person home the non-potable demand the calculator models is about 58,000 litres a year (40 l/person/day); a well-matched system might supply 30,000-50,000 litres of that, depending on roof area and local rainfall. Actual savings are capped by whichever is smaller — the rainwater your roof yields or your non-potable demand.

BS EN 16941-1:2024 (the current UK standard, which carries forward the method from the withdrawn BS 8515) recommends sizing the tank at 5% of the lesser of annual yield or annual non-potable demand. Larger tanks are deliberately discouraged because slow turnover encourages stagnation and bacterial growth. As a rule of thumb 1,000-3,000 litres suits a typical house; larger systems (5,000-10,000+ litres) suit commercial buildings. Always include a mains top-up for dry periods.

Underground tanks generally do not need planning permission as they fall under permitted development. Above-ground tanks may need permission if they alter the external appearance of the building. Check with your local planning authority, especially in conservation areas.

Annually: clean roof gutters, check filters, inspect tank and pump, test mains backup. Quarterly: check first-flush diverter. The system should include a leaf filter, first-flush diverter, and floating suction to draw clean water from below the surface.