About Rainwater Harvesting Calculator
The rainwater harvesting calculator estimates how much water a roof can collect and how large a storage tank should be. The annual yield follows the simple relation that one millimetre of rain falling on one square metre of catchment delivers one litre of water, reduced by the roof runoff coefficient and the first-flush filter efficiency.
Enter the roof plan area, the monthly rainfall, the roof type, the filter efficiency, and the monthly water demand. The tool runs a twelve-month water balance with tank carry-over, reporting the harvested supply, the demand met from rainwater, any overflow lost from a full tank, and any deficit that must be topped up from the mains, plus a recommended tank size and the overall share of demand met.
How It Works
- Enter the roof catchment area (m²) and select the roof type, which sets the runoff coefficient C.
- Enter the monthly rainfall (mm), the filter / first-flush efficiency, and the monthly water demand (litres).
- The calculator computes monthly supply = A · R · C · eff and runs a carry-over tank balance against demand.
- It reports demand met, overflow, deficit, the percentage of demand met, and a recommended tank size from the mass-curve method.
Worked Example
A roof of plan area A = 100 m² receives R = 800 mm of rain per year. With a concrete-tile runoff coefficient C = 0.85 and a filter efficiency eff = 0.90, the annual harvestable yield is V = A · R · C · eff = 100 · 800 · 0.85 · 0.90 = 61,200 litres per year (61.2 m³), because 1 mm of rain over 1 m² equals 1 litre. With a year-round demand of 5,000 litres per month (60,000 L/yr) and a 10,000 L tank, the month-by-month balance shows that almost all demand is met from rainwater, with the remainder topped up from the mains.
Formulas
- Annual harvestable yield
V_litres = A * R * C * eff- Unit relation
1 mm of rain over 1 m^2 = 1 litre (so A·R in mm·m^2 gives litres)- Monthly water balance (carry-over tank)
available = storage + supply; met = min(demand, available); storage_end = min(available - met, tank); overflow = max(0, available - met - tank); deficit = demand - met- Demand met fraction
f = total_demand_met / total_demand
Standards & References
- BS 8515 — Rainwater harvesting systems: Code of practice
- Texas Manual on Rainwater Harvesting
- ARCSA/ASPE/ANSI 63 — Rainwater Catchment Systems
- Rippl mass-curve method (storage sizing)
Frequently Asked Questions
How is the annual rainwater yield calculated?
Yield in litres equals the roof area in square metres times the rainfall in millimetres times the runoff coefficient times the filter efficiency, because one millimetre of rain over one square metre is exactly one litre. For example 100 m² × 800 mm × 0.85 × 0.90 gives 61,200 litres per year.
What is the runoff coefficient and why is it less than 1?
The runoff coefficient accounts for water lost to wetting the surface, evaporation, splash, and absorption before it reaches the tank. Smooth metal roofs are around 0.95, tiled and shingle roofs around 0.85, and extensive green roofs as low as 0.3 because they retain most rainfall.
How does the calculator size the tank?
It runs a twelve-month water balance with carry-over storage and also applies the mass-curve (Rippl) method, which finds the largest cumulative shortfall between supply and demand over the year. That deepest drawdown is the storage volume needed to bridge the dry season.
Why does a bigger tank not always meet more demand?
Once the tank is large enough to carry rainwater through the dry months, extra capacity is never filled, so it adds no benefit. If total annual supply is less than total demand, no tank size can fully close the gap and some mains top-up is always required.