Rainfall collection calculator
Estimate recoverable rainwater, tank storage and overflow for one rainfall event.
One event with no water use during rainfall. Efficiency and first flush are supplied assumptions. No forecast, plumbing design or drinking-water suitability is implied. Change the input or settings to update your result. Results update as you type. Up to 1,000,000 characters.
Your result
How to use Rainfall collection calculator
- Enter your measured quantities, units and assumptions.
- Review the calculated result and the stated limits.
- Download the report or plan for your project.
Useful next steps
Measure projected catchment area
Use the horizontal projected area and rainfall depth for the same event. Enter your collection efficiency and the first-flush volume you plan to discard.
Check the tank balance
Subtract the water already in the tank from its capacity. The result separates rain on the catchment, recoverable water, newly stored water and overflow.
Save the measurements and assumptions
Download CSV and a text or JSON report. The calculation runs on your device. Changing a unit keeps the number you entered, so check both before using the result.