About this calculator
This calculator sizes a rainwater harvesting tank for a house: how much rain the roof collects in a year, how much non-potable water you use for WC flushing, laundry and the garden, and how big a tank balances the two. The result is rounded up to the next size in a list of tanks you can edit to match a supplier's range.
The rainfall default is the 1991–2020 average for Lithuania (695 mm a year, LHMT); enter a local figure if you know it.
Formula
The simplified (annual balance) approach used with EN 16941-1:
yield Y = A × e × h × η A = roof area in plan (m²), h = rainfall (mm = l/m²)
e = yield coefficient of the roof, η = filter efficiency
demand D = persons × (WC + laundry l/person/day) × 365 + garden m² × l/m²/year
tank V = min(Y, D) × dry days / 365
Yield coefficients: smooth roof 0.9, rough pitched roof (tiles) 0.8, flat roof without gravel 0.8, with gravel 0.7, extensive green roof 0.5, intensive green roof 0.3. Filter efficiency 0.9 unless the manufacturer states otherwise.
Worked example
A 120 m² tiled roof, 695 mm of rain a year, 4 persons using 24 l/day for the WC and 10 l/day for the washing machine, 21 dry days:
yield = 120 × 0.8 × 695 × 0.9 = 60,048 l a year
demand = 4 × (24 + 10) × 365 = 49,640 l a year
tank = min(60,048; 49,640) × 21 / 365 = 2,856 l → 3,000 l tank
The roof yields more than the household uses, so the tank is sized on the demand.
FAQ
Why "the smaller of yield and demand"? A tank larger than either limit just stores water that is never used (or never arrives). The simplified method keeps enough for a typical dry spell of the smaller of the two.
How many dry days should I enter? 21 days (three weeks) is common German practice and matches the old DIN 1989-1 "6% of the annual figure"; UK practice uses 18 days (5%). A longer dry period gives a larger tank and fewer mains top-ups in summer.
Where do the default water uses come from? 24 litres per person per day for WC flushing, +10 for a washing machine and 60 l/m² a year for garden watering are the values of DIN 1989-1 (the German rainwater standard EN 16941-1 replaced). Use your own figures if you have them.
Which standard does this follow? The method is the simplified approach of EN 16941-1, but its text could not be checked clause by clause, so no clause is cited. The yield coefficients, the filter efficiency and the formula structure were checked in secondary sources quoting the standard and its draft.
Assumptions and limits
- Annual balance only: no daily simulation, no seasonal rainfall pattern. Garden watering, which is concentrated in summer, may need a larger tank than this gives.
- The tank volume is the useful volume; dead volume below the pump intake and the overflow freeboard are not added.
- Snow, first-flush diverters and evaporation are not modelled beyond the yield coefficient and filter efficiency.
- Rainwater is for non-potable use only and must be kept separate from drinking water.