Rainwater Harvesting Calculator
Estimate rainfall collection and empty-tank overflow from a roof catchment.
Your result
EVERYDAYEnter your values and select calculate.
On this page How it works & practical examples
How to use the Rainwater Harvesting Calculator
Use the horizontal catchment area corresponding to the rainfall depth. The entered collection efficiency summarizes runoff loss and first-flush assumptions. Tank capacity caps retained water in the simple no-use scenario.
The calculation
Worked example
100 m² receiving 20 mm with 80% collection yields 1,600 liters. An initially empty 1,000-liter tank retains 1,000 liters and may overflow by 600 liters.
Assumptions and limits
The tank starts empty and no water is consumed during collection. Rain timing, drainage limits and water quality are not modeled. Captured rainwater is not automatically suitable for drinking.
Common questions
Why does mm times m² give liters?
A 1 mm layer over 1 m² is 0.001 m³, which is one liter.
Can I model a partly full tank?
Use the remaining available capacity for this simple event model and account for existing water separately.