Water
Rainwater Harvesting Calculator
Estimate how much rainwater can be collected from a roof or other catchment surface.
Catchment
Represents the fraction of rainfall that becomes usable runoff. Adjust it if you have a site-specific design value.
Rainfall
Use the long-term annual rainfall for your location rather than a single unusually wet or dry year.
Collection System
%
Accounts for losses through filtration and collection components. 90% is used as an indicative default; use manufacturer or design data when available.
Water Demand
L/day
Enter only the demand you intend to supply with harvested rainwater—for example toilet flushing, irrigation or other permitted uses.
days
Used only to show a demand-based storage reference. It is not a final tank-sizing calculation.
Annual Harvesting Potential
0 L/year
Estimated rainwater reaching storage after runoff and collection losses.
Annual Harvest
0 m³
Average Monthly Harvest
0 L
Gross Rainfall on Catchment
0 L/year
Overall Capture Fraction
0%
Demand Comparison
Annual Water Demand
0 L
Theoretical Annual Coverage
0%
Average Harvest per Day
0 L/day
Demand-Based Storage Reference
0 L
Methodology:
Annual harvest is calculated from catchment area × annual rainfall × runoff coefficient × collection/filter efficiency.
This estimates annual water availability, not actual water captured by a finite tank. Real capture depends on rainfall timing, first-flush losses, tank capacity, demand pattern and overflow.
The storage reference shown, when demand is entered, is simply daily demand × selected storage days. Final tank sizing should use local rainfall distribution and a water-balance or mass-balance analysis.
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