LifeTally
Rainwater collection calculator
How much water can one rain event supply, and how much fits in my tank?
Use your horizontal collection area, rainfall depth, collection percentage and currently free tank space.
Your rain event and tank
Enter only the horizontal footprint that drains to this tank, not the sloping roof surface. Use your own overall collection percentage for all losses before storage; no typical percentage is assumed.
Enter the space currently available, not the tank's total capacity. The model assumes no water is withdrawn during this event.
How the estimate works
One millimetre of rain over one square metre is one litre. Multiply area by event rainfall, then apply your collection percentage. The tank receives the smaller of collected water and free space; the remainder overflows.
V = A × R
C = V × η ÷ 100
S = min(C, F)
O = max(C − F, 0)Assumes uniform rainfall over the connected horizontal area and a constant collection percentage for this event. Losses before storage are included only through your percentage. This is not an annual supply forecast, a tank-sizing optimiser or a hydraulic design. It does not assess water quality, treatment or suitability for use, and does not guarantee actual collection.
Texas Water Development Board: horizontal catchment area and collection losses (PDF) ↗NIST: litre and cubic metre ↗
Check the filled example
Chosen example: 50 m², 12 mm, 80% collection and 400 L of free space. Rain volume is 600 L; 480 L reaches storage, 400 L fits and 80 L overflows. These are illustrative inputs, not recommended areas, rainfall, efficiencies or tank capacities.
Accepted input values
- Connected horizontal collection area, m²
- 50
- Rainfall depth for this event, mm
- 12
- Overall collection percentage, %
- 80
- Currently free tank space, L
- 400