Collection math
Collected gallons = roof area x rainfall inches x 0.623 x collection efficiency. Overflow is collected gallons minus installed barrel storage.
RainYard Tools
Roof collection
Rain Barrel Fill Calculator
See how much water a roof section can send to one or more rain barrels during a storm.
Collected water318 galEstimated water reaching storage.
Barrel storage110 galTotal installed barrel capacity.
Overflow208 galPlan a safe overflow route.
Example
A 600 sq ft roof section in a 1 inch storm can send about 318 gallons to storage at 85% efficiency. Two 55-gallon barrels hold 110 gallons, so overflow still matters.
Overflow is part of the system
A full barrel should send water to a safe splash block, garden, drain, or overflow hose that cannot run back to the foundation.
Use lower efficiency when needed
Old gutters, leaf screens, splash loss, and first-flush diversion reduce the water that reaches the tank.
Questions
Common checks.
Will one barrel make a big dent in roof runoff?
It helps small events and watering, but one 55-gallon barrel fills quickly on a normal roof.
Should barrels be connected in series?
Linked barrels can add storage, but each one still needs stable support and overflow.
Can stored rainwater be used indoors?
This tool assumes outdoor non-potable use. Indoor use needs code-compliant treatment and local approval.
Helpful referencesEPA downspout disconnection and rain harvesting