Rain barrel or cistern: why a bigger tank often does nothing

The short answer

For a 1,500 sq ft roof watering a 200 sq ft garden, one or two barrels (110 gallons or less) deliver at least 90% of what any tank could in 2,494 of 3,206 cities (78%). A few linked barrels or a small tank is the right size in 232, and a real cistern in 453, every one of them in 7 Western states. The 27 others get so much summer rain that the garden model never runs short. Buying more storage than that mostly buys water that sits in the tank.

Why the roof is not the limit

A roof collects far more than a garden uses. In the typical city a single 55-gallon barrel lets 95% of the roof's water run over the top, and that sounds like a reason to buy more. It is not: the water only helps if the garden needs it before the barrel would have refilled anyway. In 953 barrel cities the most any tank could deliver is under 250 gallons a year, because rain already falls often enough through the summer to water the beds directly.

Large cities with 40 or more inches of rain a year where one barrel is the useful size.

Where a cistern pays off

Storage matters where rain and the garden's need fall in different seasons. In these cities a barrel fills in a winter storm and is empty weeks before summer, so it takes thousands of gallons to carry the wet season's water into the dry one.

A cistern that size is a real purchase and may need a permit and a level, load-bearing base: a full 2,500-gallon tank weighs over ten tons. Check your city's page for its tank table before pricing one.

Related: Rain in the wrong season · Gallons per inch of rain · States that cap storage.