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Rainwater Harvesting Calculator — Annual Collection
Calculate annual rainwater collection potential from your roof.
Horizontal plan footprint of the roof (length × width under the eaves), not the sloped surface — the slope is already accounted for in the runoff coefficient (BS EN 16941-1).
Sets the annual rainfall total. UK averages range from ~600 mm in the drier South East to ~1,300 mm in Wales (Met Office). Pick the region nearest your site.
Sets the runoff coefficient: pitched tiles/slate 0.85, flat roof 0.6 (conservative gravel/rough-felt figure — smooth EPDM flats run nearer 0.8), green roof 0.3 as the substrate soaks up much of the rain.
Average annual garden water consumption (default 40,000L)
Proportion of water retained after filtering (default 0.9)
Litres diverted per month for first flush cleaning (default 0)
How We Calculate This
Annual collection (L) = Roof Plan Area (m²) × Annual Rainfall (mm) × Runoff Coefficient × Filter Efficiency. Because 1 mm of rain on 1 m² is exactly 1 litre, no further unit conversion is needed (BS EN 16941-1).
Use the horizontal plan footprint of the roof, not the sloped surface — the slope is already built into the runoff coefficient.
Runoff coefficients: Pitched roof 0.85, Flat roof 0.6 (conservative figure for gravel-ballasted or rough-felt flats; smooth EPDM/metal flats run nearer 0.8), Green roof 0.3.
Regional rainfall: SE England ~600mm, Midlands ~750mm, NW England ~1000mm, Scotland ~1200mm, Wales ~1300mm (Met Office averages).
Any first-flush diversion is subtracted from the gross yield, and the result is floored at zero.
Frequently Asked Questions
A typical UK house roof (80 m² plan footprint) in the Midlands (~750 mm/yr) collects roughly 46,000 litres a year after a 0.9 filter efficiency — that is 80 × 750 × 0.85 (pitched runoff) × 0.9. The gross before filter losses is about 51,000 litres. Actual amounts vary with regional rainfall, roof type and filter performance.
Yes, rainwater is excellent for gardens. It is slightly acidic, free of chlorine, and plants prefer it. Do not use it for drinking without treatment.
For garden use only, 200-1000 litres is typical. For whole-house rainwater recycling, 2000-5000 litres. Size depends on usage pattern and dry spell frequency.
Small above-ground water butts do not need permission. Large underground tanks may require building regulations approval. Check with your local authority.
Yes, collecting rainwater from your own roof is completely legal in the UK. There are no permits needed for domestic rainwater harvesting.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.