Free rain barrel size calculator. Enter roof area and rainfall to find gallons collected per storm and standard 55 gallon barrels needed.
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Multiply roof area by rainfall inches by 0.623 gallons per square foot per inch. A 1000 square foot roof in 1 inch of rain collects 623 gallons, needing 12 standard 55 gallon barrels. The result is the theoretical storm yield before first flush and gutter losses.
Gallons collected equal roof area in square feet times rainfall in inches times 0.623, because one inch of rain over one square foot holds 0.623 gallons. In metric terms the same rule is simpler: 1 millimetre of rain on 1 square metre equals 1 litre. Use the roof footprint, the area seen from above, not the sloped surface area, because rain falls vertically and a steep roof catches no more rain than a flat roof with the same footprint.
To work out how much rainwater your roof can collect, start from these storm totals. A 500 square foot roof in 1 inch of rain yields about 311.5 gallons, a 1000 square foot roof yields 623 gallons, and a 2000 square foot roof yields 1246 gallons. Over a year, a 1000 square foot roof in 30 inches of rain theoretically sheds over 18000 gallons. These storm totals explain why one barrel never holds a whole storm: the barrel is a bucket that fills and overflows, not a season of storage.
Real systems capture about 75 to 90 percent of the theoretical figure, so plan storage at roughly 80 percent of the calculated gallons. Losses come from the first flush diverter, splash, evaporation, and gutters overflowing in heavy downpours. Roof material matters too: metal roofs shed about 95 percent of rain, while asphalt shingle roofs shed about 85 percent, so metal is the best catchment surface for harvesting.
A standard rain barrel holds 50 to 55 gallons, and it fills from only about a tenth of an inch of rain on a 1000 square foot roof. Chain two to four barrels together with a linking kit for containers and small beds, step up to a 200 to 300 gallon cistern or tote for a vegetable garden, and consider 500 gallons or more for dry climates or serious food growing. Size storage to your garden demand and your longest dry spell, not to a single storm.
A first flush diverter discards the first dirty runoff of each storm, which carries dust, pollen, bird droppings, and roof grit. A common sizing rule is about 10 gallons diverted per 1000 square feet of roof. Fit gutter screens and a fine mesh on the tank inlet to keep leaves and mosquitoes out, and keep the tank dark and sealed, because light grows algae and open water breeds mosquitoes.
Enter the roof area that drains to your barrel and a typical storm depth, then compare the storm yield with the barrels you own. Give overflow a safe place to go, such as a rain garden or swale, and never let an overflowing tank sit against a foundation. Check local rules before installing a large system, since some areas limit storage size or offer rebates, and treat harvested water before drinking it.
Standard rain barrels hold 50 to 55 gallons. Larger tanks and linked barrel systems store more, and linking barrels is cheaper than buying one huge tank.
One inch of rain on one square foot equals 0.623 gallons. Gallons equal roof area in square feet times rainfall in inches times 0.623, the standard conversion used by extension guides and the EPA method.
Real world collection is typically 75 to 90 percent of the theoretical maximum. Plan storage at about 80 percent of the calculated figure to allow for the first flush diverter, splash, evaporation, and gutter overflow.
Divide the water from a typical storm by the barrel size. One inch of rain on a 1000 square foot roof yields about 623 gallons, enough to fill about eleven 55 gallon barrels, so most gardeners chain barrels or use a cistern and overflow the rest to a rain garden.
Size storage to your roof yield and your garden demand together. A single barrel suits containers and a few beds, a 200 gallon cistern suits a vegetable garden, and dry climates need more storage to bridge longer gaps between storms.
Yes, rainwater is naturally soft and free of chlorine and fluoride, which plants like. Divert the first flush of each storm away from storage to keep dust, bird droppings, and roof debris out of your barrels.
Only with proper treatment, such as filtration and disinfection, and in line with local health regulations. Most home systems use harvested water for irrigation, toilet flushing, and laundry instead.
Use the footprint, the horizontal area the roof covers. Rain falls vertically, so a steeply pitched roof receives the same rainfall as a flat roof with the same footprint, and using sloped area overstates the yield.