Why Tank Size Matters
Selecting the right rainwater harvesting tank size is one of the most important decisions in designing an effective system. A tank that is too small will overflow frequently, wasting potential water savings and failing to deliver the return on investment you expect. A tank that is too large will cost more than necessary, take up more space, and may not fill frequently enough to keep the water fresh and well-oxygenated.
The ideal tank size achieves the optimal balance between storage capacity and cost, providing sufficient storage to meet your water needs during dry periods without being oversized for your roof catchment area and local rainfall patterns. This balance is determined by three key factors: your roof catchment area, your local rainfall patterns, and your water usage requirements.
How Rainwater Harvesting Tank Sizing Works
Rainwater harvesting tank sizing is based on a simple principle: the tank needs to be large enough to store sufficient water to bridge the gap between rainfall events. In the UK, where rainfall is relatively frequent throughout the year, tanks can often be smaller than in drier climates because the interval between significant rainfall events is typically shorter.
The calculation starts with your roof catchment area. The larger your roof, the more rainwater you can collect. We measure the footprint area of your roof, not the total surface area, since this represents the area that actually catches rainfall. A typical semi-detached house might have a roof footprint of 80 square metres, while a large commercial building could have thousands of square metres of catchment area.
Next, we consider your local rainfall. The UK receives an average of 800–1,400mm of rainfall per year, depending on location. The West of Scotland and Wales receive the most rainfall, while East Anglia and parts of the South East receive the least. We use local rainfall data to calculate the volume of water your roof can collect in an average year.
Finally, we consider your water usage. How much water do you need? A typical household might use 150 litres per day for toilet flushing, laundry, and garden irrigation. A commercial property could use thousands of litres per day. The tank needs to be sized to meet this demand between significant rainfall events.
Simple Tank Sizing Guide
For residential properties, a general rule of thumb is that the tank should hold approximately five percent of the annual rainwater yield from your roof. For a typical UK home with a 100 square metre roof in an area receiving 800mm of annual rainfall, the annual yield would be approximately 80,000 litres. Five percent of this is 4,000 litres, suggesting a tank size of around 4,000 litres for optimal performance.
However, this is a general guide and the optimal size depends on your specific circumstances. If you have a large garden with high irrigation demand, you may benefit from a larger tank. If you live in an area with frequent rainfall, a smaller tank may be sufficient. The most accurate approach is to use our detailed calculation method, which considers your specific roof area, location, and water usage patterns.
Below are typical tank size recommendations for different property types as a starting point:
Small terraced house with minimal garden: 1,500–2,000 litres. This size is sufficient for garden watering and occasional outdoor use. With limited roof area and garden space, a smaller tank is usually appropriate.
Semi-detached or detached house with average garden: 3,000–5,000 litres. This size provides sufficient capacity for toilet flushing, laundry, and garden irrigation for a typical family home. Most residential installations fall within this range.
Large detached house with extensive garden: 5,000–10,000 litres. Larger properties with higher water demand benefit from increased storage capacity. This size can meet most non-potable water needs for the household.
Commercial property or farm: 10,000–50,000 litres or more. Commercial and agricultural systems require bespoke sizing based on detailed analysis of demand and supply. Our team provides free consultations to determine the optimal size for commercial installations.
Factors That Affect Tank Sizing
Several additional factors can affect the optimal tank size for your property. The intended use of harvested water is a primary consideration. If you only plan to use rainwater for garden irrigation, you will need less storage than if you also plan to supply toilets, laundry, and other household uses. Seasonal variations in demand are also important. Garden watering demand is highest in summer when rainfall is lowest, so a larger tank is needed to store water from wetter months for use during drier periods.
Your property's location within the UK significantly affects tank sizing. Properties in the west and north of the UK, which receive higher rainfall, can often achieve good performance with smaller tanks than properties in the drier east and south east. Local rainfall patterns, including the frequency and intensity of rainfall events, should be considered in detailed sizing calculations.
Available space for tank installation may also influence your choice. Below-ground tanks can be any practical size, limited only by excavation space and budget. Above-ground tanks are more constrained by available space and visual considerations. Our team can advise on the best tank type and location for your property.
Our Detailed Sizing Service
For accurate, property-specific tank sizing, we recommend using our professional consultation service. Our trained consultants visit your property to measure roof areas, assess water usage requirements, and evaluate site conditions for tank installation. We use industry-standard calculation methods and proprietary software to determine the optimal tank size for your specific circumstances.
The consultation includes a detailed report showing estimated annual water yield, projected savings, recommended tank size, and return on investment analysis. This information enables you to make an informed decision about your rainwater harvesting investment with confidence.
Common Sizing Mistakes to Avoid
One common mistake is choosing a tank based solely on available space or budget, without considering whether it is appropriately sized for the roof catchment area and water demand. An undersized tank will overflow frequently, wasting potential water savings and reducing the return on investment. An oversized tank costs more and may not fill frequently enough, leading to stagnant water and reduced water quality.
Another common mistake is failing to account for filtration losses. Not all the rain that falls on your roof reaches your tank. First flush diverters, filtration systems, and evaporation losses typically reduce the volume collected by 10–20 percent. Our calculations account for these losses to ensure your tank is sized correctly.
Finally, many people underestimate their water usage. A typical person uses approximately 150 litres of water per day for all purposes, of which around 50 percent can potentially be supplied by rainwater. When sizing a tank for toilet flushing and laundry, it is important to use realistic occupancy and usage figures rather than optimistic estimates.
Get Professional Advice
While the guidance on this page provides a useful starting point, accurate tank sizing requires professional assessment of your specific property and circumstances. Our team of experienced consultants can visit your property, carry out detailed measurements and calculations, and recommend the optimal tank size for your needs.
Contact us today to schedule a consultation. We will provide a detailed proposal including recommended tank size, system specification, installation costs, and expected savings, all at no obligation.