This estimator calculates how much potable or purchased water a rainwater harvesting system can potentially replace over a chosen period. It combines catchment area, rainfall, collection efficiency, and usable demand so the result is constrained by both supply and actual water use. It is useful for early-stage conservation planning, utility-cost comparisons, and system feasibility checks. The calculator reports gross harvest potential, usable rainwater, and the percentage of eligible demand that could be supplied. It does not model the timing of storms or tank drawdown, so a detailed annual simulation may produce a lower usable volume when rainfall and demand are poorly matched.
1. Enter roof area
Use only roof surfaces connected to the harvesting system.
2. Enter period rainfall
Use rainfall for the same month, season, or year as the demand value.
3. Set collection efficiency
Account for runoff and system losses as a percentage.
4. Enter eligible demand
Include only uses that can be supplied by harvested rainwater.
5. Review savings and coverage
Compare usable harvested water with the eligible demand for the same period.
Efficiency is entered as a percentage and converted to a decimal. If eligible demand is zero, coverage is reported as 0%.
Estimated water savings is the portion of eligible demand that the available rainwater supply could replace during the selected period.
Actual savings can be lower if storage is too small, rainfall timing does not match demand, or water is lost to overflow, diversion, or maintenance.
Given
Area = 220 m²; annual rainfall = 800 mm; efficiency = 80%; eligible annual demand = 180,000 L.
Calculation
Harvest potential = 220 × 800 × 0.80 = 140,800 L.
Water savings = min(140,800, 180,000) = 140,800 L.
Coverage = 140,800 ÷ 180,000 × 100 = 78.22%.
Result
140,800 L
The rainwater supply could theoretically offset about 78.2% of the eligible annual demand before timing and storage constraints are considered.
No. The calculator caps savings at the eligible demand because unused harvested water does not displace additional potable or purchased water.
Rainfall and demand must cover the same period. Annual values are common for screening, while monthly values can reveal seasonal mismatches more clearly.
No. It estimates supply from rainfall volume and an overall collection efficiency but does not simulate tank level, overflow, or carryover between periods.
Yes, if irrigation is an intended use of harvested rainwater and the demand value covers the same period as the rainfall input.
A system may lose usable water because of small storage, first-flush diversion, filter backwash, leakage, maintenance downtime, or rain arriving when demand is low.