Rainwater Harvesting Calculator
Estimate annual rainwater capture, tank turnovers, storage supply days and water-cost savings from roof area and rainfall.
Default project preview
31,811 gallons
Tank supply
20 days
Usable annual water
18,250 gal
Annual water savings
$164.25
Simple payback guide
21.3 years
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Annual capture = roof area × annual rainfall depth × runoff coefficient × retained-system factor. Usable water is limited by entered annual demand.
BuildMeter Guides
Understand the measurements and assumptions
Use these visible reference guides before changing project inputs or planning allowances.
FAQ
Rainwater Harvesting Calculator questions
Why can annual capture exceed tank capacity?+
A tank can fill and empty many times during the year, so annual throughput can exceed one tank volume.
Does a larger tank always save more water?+
Not necessarily. Rainfall timing and demand determine how often storage overflows or sits empty.
Can harvested rainwater be used for drinking?+
Potable systems require treatment, testing and regulatory compliance beyond this calculator.