How Much Rainwater Can I Harvest?
Quick Answer
50,000–80,000 litres per year for a typical UK house
Based on: 80m² roof plan area, 800mm annual rainfall, 0.8 runoff coefficient = ~51,200 litres/year
How We Calculated This
The formula for annual rainwater yield:
- Yield = Roof area (m²) × Annual rainfall (mm) × Runoff coefficient
- Roof area: 80m² (plan area, not slope area)
- Annual rainfall: 800mm (UK average — varies from ~550mm in East Anglia to ~1,200mm+ in western hills)
- Runoff coefficient: 0.8 (accounts for evaporation and first-flush losses)
- Yield: 80 × 800 × 0.8 = 51,200 litres/year
What Can You Use It For?
- Garden watering: The most common use — a simple water butt is sufficient
- WC flushing: Accounts for ~30% of household water use. Requires a pump, filter, and separate pipework
- Washing machines: Possible with filtration system
- Not suitable for: Drinking, cooking, or bathing (without advanced treatment)
Storage Options
- Water butt (200–350L): £30–80. Simple garden use.
- Above-ground tank (500–1,500L): £100–400. More capacity.
- Underground tank (2,000–7,500L): £800–3,000+ installed. Required for whole-house systems.
UK Rainfall Variation
- South East England: 550–700mm/year
- Midlands: 700–900mm/year
- Wales & Lake District: 1,000–2,000mm/year
- Scotland (west): 1,200–3,000mm/year
Last updated: April 2026. Rainwater harvesting for WC flushing and washing machines must comply with BS 8515 and local water regulations.