How big of a rainwater tank do I need?

Choosing the right size rainwater tank is one of the most important decisions when setting up a harvesting system. A tank too small will overflow frequently, while an oversized tank wastes money and space. The correct size depends on collection potential, water demand, and local rainfall patterns.

Calculating Collection Potential

Every square metre of roof captures approximately one litre of water per millimetre of rainfall. The formula: roof area in square metres multiplied by annual rainfall in millimetres gives your theoretical annual collection volume in litres. A typical semi-detached house with a 50 square metre roof in a region with 800 mm of annual rainfall has a theoretical potential of 40,000 litres per year. Realistic collection efficiency is around 75-80 percent.

Calculating Water Demand

Garden watering is the most variable use — a typical garden uses between 5,000 and 20,000 litres per year. Toilet flushing uses approximately 30 litres per person per day. Laundry uses about 50 litres per load. Add up your intended uses to estimate annual demand. This figure, combined with collection potential, determines the minimum tank size needed.

Rainfall Patterns

While the UK receives rain year-round, there are dry periods — typically late spring and summer — when demand is highest and rainfall is lowest. Your tank should hold at least 5 percent of your annual demand, ideally 10 to 15 percent. A detailed water balance calculation models week-by-week collection and consumption to find the optimal size.

Typical Tank Sizes

For simple garden watering, a 200 to 500 litre water butt is usually sufficient for a small garden. Medium gardens benefit from 750 to 1,500 litres. Large gardens need 2,000 to 5,000 litres. For toilet flushing and laundry as well as garden watering, plan for 3,000 to 5,000 litres. Beyond the calculations, practical constraints like available space and budget also determine the final tank size.

Practical Constraints and Budget

Beyond the theoretical calculations, practical constraints often determine the final tank size. Space in your garden may limit the footprint or height of the tank. Above-ground tanks larger than 1,500 litres become visually prominent and may require planning permission in some areas. The weight of a full tank — one tonne per 1,000 litres — requires a suitable base and may limit placement options. Budget is also a significant factor: larger tanks cost more, and the incremental cost per litre of storage increases as tank size grows. A good approach is to start with a moderately sized tank that fits your space and budget, then add additional storage later if needed by interconnecting multiple tanks. This modular approach allows you to expand your system gradually as your needs and experience grow.

Typical Scenarios and Recommendations

For a typical UK semi-detached house with a garden of average size, a 1,000 to 2,000 litre tank provides a good balance of cost, space, and water-saving potential. This size can supply most garden watering needs and provide some water for outdoor cleaning. For a detached house with a larger garden and higher water usage, a 3,000 to 5,000 litre tank enables rainwater use for toilet flushing and laundry as well as garden irrigation. For eco-conscious households aiming for maximum water independence, 5,000 to 10,000 litres of storage combined with efficient water use can meet a significant proportion of total household water demand. Always size your tank based on your specific situation rather than generic recommendations, using a water balance calculation if possible.