How much rainwater can I collect from my roof?

A typical UK semi-detached house with 50 m² of roof can collect around 32,000 litres per year (based on 800mm average UK rainfall and 0.8 runoff coefficient for pitched tiles). This is enough for garden watering and WC flushing for a family of 4. Actual yield depends on your roof area, material and local rainfall.

What size rainwater tank do I need?

As a rule of thumb, a rainwater tank should hold approximately 5% of your annual yield or 21 days of demand, whichever is smaller. For a garden-only system in an average UK location, a 1,000-1,500 litre tank is usually sufficient. For a system serving WC flushing too, consider 2,000-3,500 litres. Our calculator works this out based on your specific roof and usage.

What is the runoff coefficient for different roof types?

The runoff coefficient represents how much rainfall actually reaches your gutters. Pitched tile or slate roofs have a coefficient of 0.8 (80% of rain collected), metal roofs are 0.9 (very efficient), flat felt roofs are 0.7 (some absorption and ponding), and green roofs are around 0.5 (they absorb significant rainfall by design).

Do I need planning permission for a rainwater harvesting system?

Underground rainwater tanks generally don't require planning permission in England and Wales as they fall under permitted development. Above-ground tanks may need permission if they are large or in a prominent position. In conservation areas, check with your local planning authority. A rainwater harvesting system connected to WC flushing must comply with Water Supply (Water Fittings) Regulations 1999.

How much annual rainfall does the UK get?

UK annual rainfall varies significantly by region: London and the South East receive around 600mm, the Midlands around 750mm, Manchester and the North West around 870mm, Edinburgh around 700mm, Cardiff and South Wales around 1,150mm, and the Lake District can exceed 1,500mm. The UK average is approximately 800mm. Our Soakaway Sizing Calculator (/landscaping/soakaway-sizing-calculator) also uses regional rainfall data.

Rainwater Harvesting Calculator — Tank Size & Yield UK

How We Calculate This

This calculator estimates the annual rainwater yield from your roof and recommends a tank size based on your household demand.

Annual yield formula

Annual Yield (litres) = Roof Area (m²) × Annual Rainfall (m) × Runoff Coefficient × 1,000

Runoff coefficients

Pitched tile / slate: 0.80
Pitched metal: 0.90
Flat felt / membrane: 0.70
Green roof: 0.50

Daily demand estimates

Garden only: ~30 litres per person per day (summer average)
Garden + WC: ~50 litres per person per day
Full non-potable: ~80 litres per person per day (includes washing machine)

Tank sizing

Tank size = smaller of (5% of Annual Yield) or (21 days × Daily Demand)
The result is rounded up to the nearest standard tank size (500-litre increments). The 5% rule prevents oversizing in high-rainfall areas, while the 21-day rule ensures adequate storage for dry spells.

For managing excess rainwater, see our Gutter Sizing Calculator (/roofing/gutter-sizing-calculator) and Soakaway Sizing Calculator (/landscaping/soakaway-sizing-calculator).

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Rainwater Harvesting Calculator — Tank Size & Annual Yield