About RoofRainYield

Why this exists

Every rain-barrel calculator prints the same flattering number: inches of rain times roof area times 0.623. That is how much water lands on the roof, not how much ends up on your garden. A 55-gallon barrel under a 1,500-square-foot roof is full after 0.06 of an inch of rain, and everything after that goes over the side. RoofRainYield answers the question people are actually asking, which is what size of tank is worth buying where they live, for 3,206 US cities with ten thousand or more people.

What goes into the number

Rainfall is the NOAA NCEI 1991-2020 U.S. Climate Normals: monthly precipitation, and the average number of days with at least a tenth and half an inch of rain, at the weather station nearest each city. City locations and populations come from the Census Bureau's 2025 Gazetteer and 2024 population estimates.

The model splits each month's rain into storms using those wet-day counts, takes off the first flush and the share a roof loses to splash and evaporation, and runs a tank of each common size through a normal year against a 200-square-foot garden that needs about an inch of water a week in the growing season. A tank size is called worth it when it delivers 90% of what the biggest tank could; past that point, extra capacity mostly sits full.

What the data shows

Across the 3,206 cities, the useful size for a typical roof and garden is one or two barrels in 2,494, a few linked barrels or a small tank in 232, and a real cistern in 453. Even the driest cities on the list get enough rain for some tank to cover a quarter of the garden's needs, but in the desert Southwest that takes a cistern, not a barrel.

State rules

Each city page shows what its state says about collecting rainwater. 12 of the 51 entries (50 states and DC) have been checked against a current statute, agency or university extension page, which is linked; the rest are marked with the date of the federal survey they come from, so you can see how old the statement is. Where a state caps storage (Colorado, Utah), the recommendation never exceeds the cap. City and county rules, HOA covenants and plumbing codes for indoor use are not covered; check them locally.

Known limits

Climate normals describe a typical year, not this one, and a dry spell can empty any tank. Storms are reconstructed from monthly figures, not measured one by one. Roofs, gutters and gardens differ: the calculator on the home page lets you change the roof size and material. Treat each page as a starting point for sizing, not a guarantee.

Corrections

If a city's rainfall or a state rule looks wrong, say so through the contact page. Confirmed errors are fixed in the data or the model, which corrects every page that depends on it at once.

Who runs this

RoofRainYield is independently run. It is not affiliated with any tank or barrel maker, retailer, installer, utility or government agency, and nobody pays to change a recommendation.