Free Online Calculators
Calculate rainwater yield & tank size
This calculator helps you estimate how much rainwater you can collect from your roof and what tank size best suits your needs. It uses your roof area, local rainfall data, and a runoff coefficient to account for evaporation and surface losses. Get a clear picture of your potential savings before investing in a rainwater harvesting system.
Start with your roof's catchment area, multiply it by the local rainfall you want to capture and subtract losses from overflow, splash and filtering. Then compare that yield with how much water you actually use in the garden over a dry spell — the tank only needs to bridge the gap between rain events, not store a whole year.
It depends almost entirely on the roof area feeding the barrel: a large roof section can fill it in a single modest shower, while a small shed roof may need several heavy downpours. Multiply the roof area by the rainfall depth to get the volume, then subtract roughly a tenth for losses.
Size it around the longest dry period you want to cover, multiplied by your daily water demand. Garden-only users typically need far less storage than households using rainwater for toilets and laundry, and a tank larger than your roof can refill is wasted money.
Multiply the horizontal roof area by the rainfall depth for the period, then apply a runoff coefficient for the roof material and a small deduction for the filter. Tiled and metal roofs lose less than gravel or green roofs.
Rules vary by country, state and even municipality — some places encourage unlimited harvesting, others cap barrel volume or require a permit for larger tanks and any connection to indoor plumbing. Check with your local water authority or building department before installing.
Compare monthly rainfall yield from your roof against monthly water demand, and pick the storage volume that keeps the tank from running dry during the driest stretch. A simple month-by-month balance calculation shows where extra capacity stops adding useful water.
Larger tanks generally need a level, load-bearing base, and a reinforced concrete slab is the standard solution because water is extremely heavy. Small barrels can sit on compacted gravel or paving slabs, as long as the surface is flat and stable.
Work out annual roof yield from catchment area, rainfall and runoff losses, then estimate annual demand from garden watering and any indoor uses. Storage is normally set to cover a few weeks of demand, sized to whichever is smaller — supply or demand.
Polyethylene tanks typically last a couple of decades if shielded from direct sunlight, while concrete and properly coated steel tanks can serve considerably longer. Lifespan depends mainly on UV exposure, water quality and how well the base and fittings were installed.
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.