Rainwater Harvesting Calculator – Roof Yield & Tank Sizing
Use the rainwater harvesting sizer to perform a quick preliminary calculation from project dimensions and engineering assumptions. Review inputs and verify final results against project requirements.
Introduction
Estimate rooftop rainwater yield and preliminary storage requirements using catchment area, rainfall and runoff assumptions.
Rainwater Harvesting Formula
A simplified annual or event-based yield can be estimated as Rainfall × Catchment Area × Runoff Coefficient. Units must be made consistent before conversion to litres.
Runoff Coefficient
Not all rainfall becomes collectable runoff. Roof material, slope, surface condition and losses influence the runoff coefficient.
Tank Sizing
Storage should be matched to rainfall distribution, demand, overflow arrangements and available space. A single annual-yield number does not automatically determine the optimum tank volume.
Worked Example
If rainfall is 800 mm, catchment area is 100 m² and runoff coefficient is 0.8, approximate annual yield is 0.8 × 100 × 0.8 = 64 m³, or 64,000 litres.
FAQs
Q: Can all roof water be stored? A: No. First-flush diversion, evaporation, overflow and collection losses may reduce usable water.
Q: Is annual rainfall enough for tank sizing? A: Monthly rainfall and demand are more useful for storage planning.
Disclaimer
Final rainwater systems should account for local rainfall data, demand, overflow, filtration, structural capacity and applicable plumbing/environmental requirements.