Rainwater Harvesting Calculator — Tank Size & Annual Yield

A typical UK semi-detached house with 50 m² of roof can collect around 28,800 litres per year (based on 800mm average UK rainfall, a 0.8 yield coefficient for pitched tiles and the 0.9 filter/hydraulic efficiency coefficient required by BS EN 16941-1:2024). 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.

Following the BS EN 16941-1:2024 (formerly BS 8515:2009) simplified rule, a rainwater tank should hold the lesser of 5% of your annual yield and 5% of your annual demand (about 18 days of demand). For a typical UK roof, 5% of annual yield is usually the limiting term, so the standard caps most domestic tanks at roughly 1,000-1,500 litres whether the system feeds the garden only or also WC flushing — a deliberate limit that prevents stagnation. Larger tanks (2,000-3,500 litres) are only justified where you have a big collecting roof or unusually high demand. Our calculator applies the BS rule to your specific roof and usage.

The runoff (or yield) 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). A separate filter/hydraulic efficiency coefficient of 0.9 (per BS EN 16941-1:2024) is then applied for losses through the pre-tank filter.

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.

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 also uses regional rainfall data.