Rain Barrel Calculator

Multiply your roof's catchment area in square feet by the rainfall in inches, by 0.623, by a runoff coefficient of roughly 0.8 to 0.9 depending on roof material. A 1,200 square foot roof section in a 1 inch storm at a typical 0.8 coefficient collects about 598 gallons, enough to fill nearly twelve standard 50 gallon barrels. Enter your own roof area and rainfall above for an exact figure.

Less than you'd think. A standard 50 gallon barrel fed by a 500 square foot roof section fills after about a fifth of an inch of rain at a typical 0.8 runoff coefficient, since 500 times 0.2 times 0.623 times 0.8 is roughly 50 gallons. Most single storms exceed that easily, which is why overflow, not underfilling, is the normal outcome for a rain barrel.

Gallons collected equals roof area in square feet, multiplied by rainfall in inches, multiplied by 0.623, multiplied by a runoff coefficient. This is the formula published in the Texas Water Development Board's Texas Manual on Rainwater Harvesting. The 0.623 is exact geometry, not a measured figure: one inch of rain on one square foot is 144 cubic inches, and a US gallon is 231 cubic inches, so 144 divided by 231 is 0.6234.

0.623 gallons per square foot for every inch of rain, before accounting for runoff losses. Multiply that by your roof's runoff coefficient, roughly 0.8 for asphalt shingle up to 0.9 for metal, to get the realistic collected figure rather than the theoretical maximum.

Almost certainly, in any storm heavier than a fraction of an inch. A modest 1,200 square foot roof section in just a 1 inch storm produces close to 600 gallons, far more than a single 50 gallon barrel holds. Enter your roof area, a single storm's rainfall, and how many barrels you have above to see the specific overflow amount for your setup.

It depends on how much overflow you're willing to accept in a normal storm, since one or two 50 gallon barrels per downspout is standard practice and will still overflow in anything beyond light rain. This calculator shows the overflow gallons for your specific roof area, storm size and barrel count, so you can see whether adding a second daisy-chained barrel meaningfully closes the gap or whether you actually need whole-system storage instead.

50 gallons is the common retail size sold at most hardware and garden stores, and it's the default this calculator uses, though the barrel size field is editable if yours is larger or smaller.

Yes, meaningfully. A smooth metal roof runs close to a 0.9 runoff coefficient, shedding nearly everything that lands on it, while a more porous asphalt shingle roof runs closer to 0.8 because more water is absorbed or lost to splash before reaching the gutter. That gap is worth roughly 10% of your total collection, which is why this calculator asks for roof type rather than assuming one fixed efficiency for every roof.

0.623 gallons per square foot of roof, before runoff losses. For a typical 1,500 square foot roof at a 0.8 runoff coefficient, 1 inch of rain works out to about 748 gallons collected, roughly fifteen standard 50 gallon barrels' worth.

Roughly the same math as a 50 gallon barrel, just with a slightly higher target: enter 55 in the barrel size field above rather than the default 50, along with your roof area and rainfall, and the calculator scales the barrel count accordingly.

Scale and framing, not the underlying formula. A rain barrel is a small, opportunistic catch, typically 50 gallons, sized around a single storm and expected to overflow in anything beyond light rain. A rainwater harvesting system is a cistern sized against your annual rainfall and your actual outdoor water demand, meant to supply irrigation for days or weeks rather than catch one storm's first flush. Use this calculator for the single-barrel, single-storm question, and the Rainwater Harvesting Calculator for sizing a bigger system against a season of use.

Routed away from the foundation, the same place you'd want a downspout extension to discharge, since an overflowing barrel next to the house is functionally the same problem as a downspout dumping water against the foundation. This calculator doesn't size overflow piping, but it does flag when a storm exceeds your barrel capacity so you know an overflow path is doing real work, not just sitting there as a precaution.