Rainwater Harvesting Calculator
Estimate how much rainwater your roof can collect each year.
Results update as you type.
How it works
Rainwater harvesting captures the rain that falls on your roof and stores it for watering gardens, flushing toilets, or washing, cutting the mains water you buy. This rainwater harvesting calculator estimates how much water your roof could collect in a year from three inputs: the roof's catchment area, your local annual rainfall, and a collection efficiency that accounts for real-world losses. The physics is elegantly simple — a millimetre of rain on a square metre yields a litre of water — so a large roof in a wet climate can gather tens of thousands of litres a year. The efficiency figure, defaulting to 80 percent, reflects that gutters, filters, first-flush diverters, and evaporation prevent you from capturing every drop. Knowing your potential yield helps you size a tank, judge whether harvesting is worthwhile, and estimate the mains water and money you could save. In regions with water restrictions or high tariffs, the savings can be substantial, and stored rainwater reduces stormwater runoff that would otherwise strain drains. Homeowners planning a rain barrel or cistern, gardeners wanting free irrigation, and self-builders designing water systems all use a rainwater harvesting calculator to plan capacity before installing anything. Match the collected figure against your intended use to see how many months of the year rainwater could meet your needs.
litres/yr = roof area (m²) × rainfall (mm) × efficiency% ÷ 100
One millimetre of rain falling on one square metre of roof delivers exactly one litre of water, which makes roof area in square metres multiplied by annual rainfall in millimetres a direct estimate of the litres landing on the roof. Not all of that water reaches your tank, so multiplying by a collection efficiency and dividing by 100 accounts for splashing, evaporation, first-flush diversion, and gutter losses, giving a realistic figure for the water you can actually store and use.
Source:Standard rainwater catchment formula
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter roof catchment area, annual rainfall and collection efficiency.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Water collected updates instantly as you type — no button to press.
Frequently asked questions
How do I measure my roof's catchment area?
Use the roof's footprint — its horizontal plan area — not the sloped surface, because rain falls vertically and a pitched roof catches the same rain as its flat shadow on the ground. Measure the building's length and width at the eaves and multiply them. If only part of the roof drains to your collection point, count just that portion. For complex roofs, add up the footprints of each section that feeds your tank.
What collection efficiency should I use?
Around 80 percent is a sensible default for a well-set-up system with gutters and a first-flush diverter. Smooth metal roofs collect efficiently, while rough or porous surfaces, heavy tree cover, and evaporation lower the figure. First-flush devices that discard the initial dirty runoff also reduce yield slightly. If your system is basic or your roof is textured, use 70 percent; a clean, optimised metal roof might reach 85 to 90 percent.
Where do I find my annual rainfall figure?
National meteorological services and weather websites publish average annual rainfall in millimetres for most locations, often searchable by town or postcode. Use a long-term average rather than a single unusual year for planning. Remember rainfall is seasonal, so a yearly total does not mean steady supply — you may collect most of your water in a few wet months, which affects how large a storage tank you need.
How big a tank do I need for this much water?
Tank size depends on how evenly rain falls and how steadily you use the water, not just the annual total. A tank need not hold a whole year's collection; it bridges dry spells between rains. In a climate with regular rain, a few thousand litres may suffice, while long dry seasons demand far larger cisterns. Match tank capacity to the longest gap between useful rainfalls and your daily demand.
Is harvested rainwater safe to drink?
Not without proper treatment. Roof runoff can carry dust, bird droppings, debris, and contaminants from roofing materials, so untreated rainwater suits non-potable uses like gardens, toilets, and washing. Making it safe to drink requires filtration and disinfection such as UV treatment or boiling, plus suitable roofing and regular tank maintenance. Check local regulations too, as some areas restrict or require permits for potable rainwater systems.