Rainwater harvesting calculator
One millimetre of rain on one square metre of roof is exactly one litre, less whatever the roof and filter lose.
mm
%
Tile or slate 0.85; flat gravel 0.6; green roof 0.3.
%
days
per m3
Water collected per year
45 900L
- Average per day
- 125.7
- Share of demand met
- 83.8 %
- Suggested tank size
- 3150
- Storage for the dry spell
- 3150
- Annual mains water saving
- 96.39
- Average per month
- 3825
- Litres per millimetre of rain
- 61.2
Show the working
- Roof plan area = 80
- Annual rainfall = 750
- Runoff coefficient = 85
- Filter efficiency = 90
- Daily demand = 150
- Longest dry spell = 21
- Mains water price = 2.1
- Water collected per year = 80 * 750 * 85/100 * 90/100 = 45 900
- Average per day = 45 900 / 365.25 = 125.667
- annual_demand = 150 * 365.25 = 54 787.5
- Share of demand met = 54 787.5 > 0 ? 45 900/54 787.5*100 : 0 = 83.7782
- Storage for the dry spell = 150 * 21 = 3150
- Suggested tank size = min(3150, 45 900 / 365.25 * 30) = 3150
- Annual mains water saving = min(45 900, 54 787.5) / 1000 * 2.1 = 96.39
- Average per month = 45 900 / 12 = 3825
- Litres per millimetre of rain = 80 * 85/100 * 90/100 = 61.2
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- collected = roof area x rainfall x runoff coefficient x filter efficiency
- one millimetre over one square metre is one litre exactly
Use the plan area of the roof, not the sloping area. Rain falls vertically.
A tank much larger than about a month of average collection rarely pays for itself; it just sits empty or full.