Garden & Water Utilities
Rainwater Tank Sizing Calculator
Estimate how much rain a roof can collect over a year and choose a starting tank size for the dry spell you enter. Roof area, rainfall, efficiency and demand are all editable.
Catchment is the first sizing check
The annual harvest estimate follows the Texas Water Development Board's basic relationship: rainfall × catchment area × collection efficiency. The tank recommendation here is a dry-spell reserve based on daily demand; local rainfall timing, overflow, first-flush diversion, roof material, water restrictions and non-potable-use rules can change the final design.
Source: Texas Water Development Board, Rainwater Harvesting Potential and Guidelines.