Free Online Calculators
Calculate rainwater yield & tank size
Collecting rainwater is one of the smartest ways to reduce your water bills, protect the environment, and keep your garden thriving even during dry spells. Our free Rainwater Tank Calculator helps you quickly determine how much rainwater your roof can harvest each year and what tank size best suits your needs. Whether you want to water your garden, flush toilets, or run your washing machine on rainwater, this tool gives you the numbers you need to plan your system confidently.
The calculator uses three key inputs to estimate your rainwater yield and recommend an appropriate tank size. Simply enter your roof catchment area, the runoff coefficient for your roof material, and the average annual rainfall for your location. The tool instantly calculates the theoretical annual yield in litres and suggests a tank capacity that balances supply and demand.
Rainwater harvesting has clear environmental and financial advantages that make it worth considering for almost any property owner:
Consider a typical detached house in London with a projected roof area of 90 m², concrete roof tiles (runoff coefficient 0.78), and an average annual rainfall of 600 mm. The calculation gives: 90 × 0.78 × 600 = 42,120 litres per year, or around 115 litres per day. If the household uses rainwater only for toilets and garden irrigation (approximately 60 litres/day), a 3,000–5,000 litre tank would provide excellent coverage throughout the year, with enough buffer for a 30–50 day dry spell.
Tank size depends on both your annual yield and your intended use. As a general guide: Garden irrigation only typically requires 1,000–3,000 litres. Garden plus toilets calls for 3,000–6,000 litres. Full household use including laundry benefits from tanks of 6,000–15,000 litres or more. The calculator factors in your daily demand and recommends a size that provides a 30–60 day buffer against dry periods, which is the industry standard for reliable supply.
Above-ground tanks (butts and barrels) are inexpensive and easy to install but limited in capacity, typically 100–1,000 litres. Underground tanks or cisterns offer much larger capacities (3,000–20,000+ litres), keep water cool to prevent algae growth, and are discreet. For year-round household use, an underground system is strongly recommended.
The accuracy of your results depends heavily on using reliable rainfall figures. In the UK, the Met Office provides regional rainfall averages. In the US, NOAA offers detailed precipitation maps. In Australia, the Bureau of Meteorology (BOM) publishes location-specific data. Always use the long-term average annual rainfall for your specific area rather than a national average for the most realistic estimate.
The calculated yield is a theoretical estimate based on average conditions. In practice, account for a 10–20% loss due to evaporation, overflow during heavy storms, and system downtime for maintenance. The runoff coefficient in the calculator already accounts for some of these losses.
Use 0.75–0.80 for clay or concrete tiles, 0.85–0.90 for metal, slate, or asphalt roofing, and 0.15–0.50 for green or living roofs depending on substrate depth and plant coverage. If you are unsure, a conservative value of 0.75 is a safe starting point.
In most countries, untreated rainwater is not approved for drinking without certified filtration and disinfection systems. It is legally and safely used for toilet flushing, garden watering, and laundry in most jurisdictions. Always check your local regulations.
Requirements vary by country and region. In the UK, most domestic rainwater systems do not require planning permission, but systems connected to mains water must meet specific standards to prevent cross-contamination. Always consult your local water authority before installation.
Experts recommend inspecting your tank annually and cleaning it every 2–3 years to remove sediment and prevent bacterial growth. First-flush diverters and leaf guards on gutters significantly reduce contamination and maintenance needs.