Rainwater Harvesting Calculator
Calculate annual rainwater collection potential from your roof. Determine tank sizing and whether collection meets your water needs.
Inputs
sq ft
inches
%
gal
gal/day
Results
Annual Collection
26,166 gal
Days of Storage
50 days
Monthly Average2,181 gal
Annual Surplus/Deficit7,916 gal
First Flush Volume15 gal
How to Use This Calculator
- Measure your Roof Area Sq Ft — the catchment surface — and look up your Annual Rainfall (inches) from local weather records.
- Set Collection Efficiency (%) accounting for first-flush diversion, evaporation, and roof material losses (typically 80–90%).
- Enter Storage Tank Gallons and Daily Usage Gallons to calculate how many days of supply your tank provides.
- Review Annual Collection (gal) and Monthly Average to assess whether rainfall matches your seasonal demand.
- Check Days of Storage to evaluate whether your tank is sized for drought periods or just supplemental use.
- Use Surplus/Deficit (gal/yr) and First Flush Volume to design overflow routing and pre-tank filtration.
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