Roof-Based Estimate
Calculate collection using the actual horizontal catchment area of your roof.
Water Collection Planning Tool
Rainwater Harvesting Calculator
Estimate how much rainwater you can collect from your roof. Enter your roof area, rainfall, runoff coefficient, collection efficiency, and tank size to receive a detailed harvesting estimate.
Calculate Rainwater Collection
Enter your catchment and rainfall information below.
What Is a Rainwater Harvesting Calculator?
A rainwater harvesting calculator estimates the volume of water that can be collected from a roof or another suitable catchment surface. It combines rainfall depth, catchment area, runoff coefficient, and system efficiency to produce a realistic collection estimate.
The result can help homeowners, gardeners, builders, farmers, and sustainability planners select an appropriate storage tank and understand the potential value of a rainwater harvesting system.
Rainfall Adjustment
Use annual rainfall or the total rainfall for a specific month, season, or storm.
Practical Loss Factors
Account for roof runoff, first-flush diversion, filters, gutters, evaporation, and leakage.
How the Rainwater Harvesting Calculator Works
Enter the Roof Area
Measure the roof catchment area in square metres or square feet. Use the horizontal roof footprint rather than the sloped roof surface where possible.
Add the Rainfall
Enter total rainfall in millimetres or inches. Annual rainfall is useful for long-term estimates, while storm rainfall helps calculate event-based collection.
Adjust System Losses
Choose a runoff coefficient and efficiency percentage to reflect the type of roof and expected losses from the collection system.
Rainwater Harvesting Formula
For metric calculations, one millimetre of rainfall over one square metre of roof area equals approximately one litre of water before losses.
Metric formula
Collected litres = rainfall in mm × roof area in m² × runoff coefficient × system efficiency.
Imperial formula
Collected US gallons = rainfall in inches × roof area in ft² × 0.623 × runoff coefficient × system efficiency.
| Roof material | Suggested runoff coefficient | General guidance |
|---|---|---|
| Metal roof | 0.90 to 0.95 | Smooth surface with relatively low absorption. |
| Tile roof | 0.80 to 0.90 | Some water may remain between tiles or surface joints. |
| Asphalt shingles | 0.75 to 0.90 | Collection performance varies with roof age and surface condition. |
| Concrete roof | 0.70 to 0.85 | Porous surfaces may absorb more initial rainfall. |
| Green roof | 0.30 to 0.60 | Vegetation and growing material retain a larger portion of rainfall. |
Runoff coefficients are approximate. Roof slope, condition,
rainfall intensity, local climate, gutter design, and first-
flush systems can change actual water collection.
Rainwater Harvesting Calculator FAQs
The amount depends on your roof area, rainfall, roof
material, gutter performance, filters, and storage
capacity. In metric units, one millimetre of rain over
one square metre produces approximately one litre before
system losses.
Enter the horizontal footprint of the roof section that
drains into your collection system. Do not include roof
sections connected to different gutters or downpipes
unless they also feed the same tank.
A runoff coefficient represents the portion of rainfall
that flows from a catchment surface. A coefficient of
0.85 means approximately 85% of the rainfall reaches the
collection system before additional system losses.
A well-maintained system may operate at around 85% to
95% efficiency. Lower values may be more suitable when
gutters overflow, filters are restrictive, or a large
first-flush volume is diverted.
The calculator shows the total potential collection and
estimates how many times the selected tank could be
filled. Actual storage depends on water use, rainfall
timing, overflow, and the amount already stored.
Potable use may require treatment, testing, filtration,
disinfection, approved materials, and compliance with
local regulations. Untreated rainwater is commonly used
for irrigation, toilet flushing, and outdoor cleaning.