What could your
roof collect?

Measure the horizontal footprint of the roof feeding your system, including overhangs. Use length × width; roof slope does not add collection area.

Collection allowance 70% collected · 30% loss 80% collected · 20% loss 90% collected · 10% loss 100% · no losses

80% is a starting assumption for water lost at the roof and gutters. Adjust it for your system; actual collection varies.

Square feet × inches of rain × 0.623 × collection allowance = US gallons.

Annual rainfall gives a yearly supply estimate, not a tank size. Storage also depends on rainfall timing, available space, and landscape water use. This estimate does not subtract tank overflow.

For landscape planning. Stored rainwater requires treatment before drinking.

Learn more: University of Arizona · BlueBarrel sizing guide