Rainwater Harvesting Tank Sizer
Enter your roof size, pick your nearest city to load historical rainfall data, tick the water uses you want to cover, and get a recommended tank size, first-flush diverter volume, and estimated annual savings on your water bill. Runs entirely in your browser - nothing uploaded.
Units
Roof
Rainfall
Non-potable water demand
Savings & Payback
Recommended tank size
Payback
Tip: a first-flush diverter is inexpensive (<£20) and significantly improves water quality. Size it to the volume shown above.
Monthly supply vs demand
Rainwater harvesting system sizing and planning
How tank size is matched to rainfall and seasonal water need
Tank size determines how much water you can store during heavy rain and how long you can draw on reserves during dry periods. The calculator compares your monthly average rainfall (derived from your zip code or entered manually) against your monthly water demand (irrigation, household, livestock). Months with surplus rainfall fill the tank; months with deficit draw it down. Tank capacity must be large enough to carry you through your driest consecutive month(s). For example, in a climate with rain from October to April and no rain May through September (like Mediterranean), a 6-month dry season means you need a tank that can hold 6 months of demand. The calculator estimates how full your tank would be each month and suggests a size that prevents overflow while avoiding regular shortfalls.
Rainfall collection area and runoff coefficient
Rainwater collection is calculated as roof or catchment area × rainfall depth × runoff coefficient. A "runoff coefficient" accounts for losses - typical roofing loses about 5-10% to evaporation and splash, and first-flush systems (which discard the first rain to clean the roof) add another 10-50% loss depending on how much dirty water you want to exclude. The calculator uses conservative assumptions; your actual yield may vary based on roofing material (metal roofs yield ~95%, asphalt shingles ~85%), climate humidity, and roof angle. Steeper roofs and smooth materials capture more. Measure or estimate your actual roof area in square meters or feet for the most accurate calculation.
FAQ
How much rainfall does my area get, and should I install a rainwater system?
Use the calculator's zip-code lookup to see your annual rainfall. Areas with less than 600mm/year may not justify large tanks unless you have very high water costs or severe scarcity. Areas with 800mm+ and seasonal dry periods are excellent candidates. Check local regulations - some regions restrict rainwater harvesting, and you may need permits.
What happens if my tank overflows?
Overflow is normal and means your tank is full. Excess water can be directed to a storm drain, landscaping, or a secondary tank. Overflow is good - it means you have adequate capacity. Problems arise when tank underflows (runs dry), which is why the calculator sizes conservatively for worst-case months.
Is rainwater safe to drink without treatment?
Rainwater is not safe to drink untreated. Roofs accumulate dust, bird droppings, and debris. A first-flush system and fine filters (20 micron) reduce contaminants, but for drinking water use boiling, ultraviolet treatment, or chlorination. For irrigation and toilet flushing, minimal treatment is acceptable. Check local health department regulations for your intended use.