Rainwater Collection Calculator
Estimate captured rainfall, storage used and potential overflow from a catchment.
What would you like to do next?
Convert rainfall depth across a catchment into volume
Enter roof or catchment area, rainfall depth, runoff efficiency and storage capacity in liters. The calculator converts either square meters with millimeters or square feet with inches into gross water, applies the collection allowance and shows the portion that fits in storage plus potential overflow. The output also converts captured liters to US gallons.
Account for water quality and system losses
Wind, splash, first flush, roof material, gutters, screens, leaks, debris and tank freeboard affect usable yield. Stored rainwater can require treatment and separation from drinking systems, and collection rules vary. Confirm structural loads, overflow drainage, mosquito control, freezing protection and local health or plumbing requirements.
Frequently asked questions
Why does one millimeter over one square meter equal one liter?
That depth and area form a volume of one thousandth of a cubic meter, which is one liter.
Does the tool subtract a first flush?
No. Represent expected losses through runoff efficiency or reduce the rainfall input.
Is captured water safe to drink?
The calculator does not assess or treat water quality.