Water and sewage bill
Calculate your water and wastewater bill from meter consumption in cubic metres (m³) and local utility tariffs.
Open calculatorRainwater tank
The yearly rainwater yield is roof area (m²) × yearly rainfall (mm) × runoff coefficient — 1 mm of rain on 1 m² is 1 litre. Rainwater replaces mains water only up to your demand (litres per day × days of use per year); the surplus goes to the overflow. The recommended tank volume is 6% of the smaller of yearly yield and yearly demand (the simplified DIN 1989-1 method, about three weeks of reserve), rounded up to 100 l. Yearly savings are the m³ used × the water and sewage price, and the payback is tank cost ÷ yearly savings.
Roof area in horizontal projection (not the sloped surface) that drains through gutters into the tank.
m²
Example: 600 mm is common in central Europe. Check the long-term average for your area with your national weather service.
mm
Picking a covering fills in an approximate runoff coefficient. These are example values — you can change them in the field below.
Share of rainfall that reaches the tank (0 to 1). The rest is held by the roof, evaporates or is lost at the filter. Example: 0.8 for tiles.
Litres of rainwater you use on a day of use. Example: one WC flush takes about 4–9 l; watering a garden takes a few litres per m² a day.
l/day
Enter 365 for a WC. If the rainwater is only for the garden, enter the season length, e.g. 150 days (May–September).
Example combined rate per m³ of water and sewage (a Polish tariff in PLN) — enter your utility's current price in your currency. With a separate garden meter, enter the water price only. Rainwater used in a WC (toilet) enters the sewage system, so your municipality or water utility may charge a sewage fee for it; in that case, savings apply only to the water price (enter the water price only).
/m³
Example amount — enter the price from your quote (tank, filter, pump, installation) or leave it empty to skip the payback.
Rainwater collected per year
57,600 labout 57.6 m³ a year
Recommended tank volume
3,500 l
Share of demand covered by rainwater
78.9%
Yearly savings
921.60 PLN
Tank payback time
6.5 years
Yearly demand
73,000 l
Rainwater used
57,600 l
Surplus to overflow
0 l
Yearly model: it ignores how rain is spread over the year (droughts, winter), breaks in use during frost, pump electricity and filter servicing. Rainfall, runoff coefficient, water price and tank cost are examples to check, not current rates. If rainwater flushes the WC, your utility may still charge sewage on it, which lowers the savings.
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Frequently asked questions
How much rainwater can a roof collect?
Every millimetre of rain on 1 m² of roof is 1 litre of water. The yearly yield is therefore roof area in horizontal projection × yearly rainfall × runoff coefficient. Example: 120 m² × 600 mm × 0.8 = 57,600 litres, about 57.6 m³ a year. If you use less than that, the surplus goes to the overflow.
How big should a rainwater tank be?
The calculator uses the simplified method of the German standard DIN 1989-1: the volume is 6% of the smaller of yearly rainwater yield and yearly demand. That is roughly three weeks of reserve without rain. The result is rounded up to 100 litres; in practice pick the next larger tank a manufacturer offers.
What is the runoff coefficient and which value should I use?
The runoff coefficient is the share of rainfall that actually reaches the tank. It is high on metal (about 0.9), about 0.8 on tiles, about 0.6 on a gravel flat roof and only 0.3–0.5 on a green roof, because plants and substrate hold water back. These are approximate values — filter and gutter losses can lower them further.
How much can I save with rainwater?
The savings are the rainwater that replaces mains water (the smaller of yield and demand) in m³ multiplied by the water and sewage price. With a separate garden meter you usually save only the water price. If rainwater flushes the WC, your utility may still charge sewage on it, so real savings can be lower. Enter the rates from your own tariff.
How long does a rainwater tank take to pay for itself?
The simple payback is the installed tank cost divided by the yearly savings. The calculator ignores grants, water price changes, pump electricity and filter servicing. If you leave the cost empty, no payback is shown, and with a water price of zero the tank never pays for itself.
Educational estimate only. Not medical, tax, legal or financial advice.