What this tool does
Make the roof-area and rainfall relationship visible while separating runoff, collection and fixed-loss assumptions.
When to use it
Use with local rainfall for the same period as the demand being assessed. Follow it with a storage simulation because annual yield alone does not show dry-season reliability.
Inputs and supported units
Each labelled field explains the quantity it expects. Unit-system changes convert existing numeric entries once and keep calculations on a single SI internal basis. Supported shared units include L/min and GPM, metres and feet, kPa and psi, millimetres and inches, kW and hp, and °C and °F as relevant.
Calculation or decision method
In SI, 1 mm on 1 m² equals 1 litre.
How to interpret the result
Read the primary result with the supporting breakdown and warning. A plausible number is not evidence that every input, operating case or design constraint has been included.
Worked example
A 150 m² horizontal roof receiving 80 mm, with 0.9 runoff, 0.85 collection efficiency and 100 L fixed loss, yields about 9,080 L.
Assumptions
Area is the horizontal plan area draining to the tank; rainfall covers the chosen period; coefficient and efficiency are dimensionless fractions.
Limitations
Even monthly yield does not represent event timing, tank overflow, drought sequence, roof contamination or end-use quality.
Sources
- U.S. Department of Energy — Rainwater Harvesting Tool Help Guide
- Australian Government — YourHome Rainwater Guidance
- U.S. CDC — Collecting Rainwater and Your Health