Rain Barrel & Cistern Sizing Calculator
Enter your roof catchment area and local rainfall to estimate how much water you can catch in a year, and what size tank to install.
Collection = roof area × annual rainfall × 0.623 gal/sqft/in × efficiency. State rainfall is the NOAA/NCEI 1991-2020 statewide average; enter your local value for accuracy. The recommended tank stores about two weeks of garden use (capped at 25% of annual collection) — a practical above-ground cistern size.
Rainfall source: NOAA/NCEI 1991-2020 climate normals — statewide average annual precipitation (inches). Local values vary; verify with your nearest station.
Frequently asked questions
How much rain can my roof actually collect?
Multiply your roof catchment area (sq ft) by annual rainfall (inches) by 0.623 gallons per sq ft per inch, then by your collection efficiency (often ~0.8). Screened, guttered roofs lose some to splash and overflow, which the efficiency factor accounts for.
What size tank should I buy?
We recommend a practical above-ground size that stores about two weeks of garden use (capped at 25% of annual collection). Bigger cisterns capture more overflow but cost more; in wet states a smaller barrel is often enough for supplemental irrigation.
Are state rainfall numbers exact for my address?
No. They are NOAA/NCEI 1991-2020 statewide averages. A single roof can differ by inches from the state mean, so use the override field to enter your nearest weather-station or utility value for the most accurate result.