What can rainwater be used for?

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.

How much water can I save?

A typical UK household can save 30-50% of mains water by using rainwater for WC flushing, washing machines, and garden use. For a 4-person household, this equates to approximately 50,000-80,000 litres per year, saving on water bills and reducing demand on mains supply.

What size tank do I need?

BS 8515 recommends a tank equal to 5% of the lesser of annual supply or demand. 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.

Do I need planning permission for a rainwater tank?

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.

What maintenance does a rainwater system need?

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.

TradeCalculator

Rainwater Harvesting Tank Calculator

Size a rainwater tank from roof area, rainfall, and demand

Roof Collection Area

Roof Material

UK Region

How We Calculate This

Enter your roof collection area, roof material (which determines the runoff coefficient), and UK region (which determines annual rainfall). The calculator computes annual rainwater supply. Enter daily non-potable water demand per person and number of occupants to determine annual demand. Tank size is calculated using BS 8515 (5% of lesser value) and a buffer storage approach, recommending the larger result.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.