Rain Barrel Calculator

It is a unit conversion, not a fudge factor. One cubic foot of water equals 1,728 cubic inches, and one US gallon equals 231 cubic inches, so 1 cu ft holds 1,728 / 231 = 7.48 gallons. One square foot of roof under one inch of rainfall captures 1/12 of a cubic foot of water, which works out to 7.48 / 12 = 0.6234 gallons. That is the theoretical maximum before you subtract any losses. A 1,000 sq ft roof in a 1 in storm could deliver about 623 gallons of water if you collected every drop.

Rain falls vertically, so the catchment area is the horizontal projection of your roof. A steeper pitch increases the slope length, but it also lets less rainwater land on each square foot of slope, and the two effects cancel out exactly. The flat outline you see from above (the building footprint) is what determines how much rainwater hits the roof. Use the same footprint number whether your roof is flat, 4/12, or 12/12; the harvest does not change with pitch.

Real roofs lose 10 to 25 percent of the theoretical harvest before water reaches the barrel. Wind blows light rain off the surface. Asphalt shingles absorb some water and release it slowly as evaporation. Gutters miss roughly 10 percent of the flow at the eaves, especially in heavy storms. Many systems include a first-flush diverter that dumps the first 5 percent of runoff to wash off dust and bird droppings, which is good for water quality but a real loss. 90% is a typical clean-gutter, no-diverter best case; drop to 75 to 80 percent if you have first-flush diversion or older gutters.

Rules vary by state and local jurisdiction. Colorado allows qualifying residences to use up to two rain barrels with a combined 110-gallon capacity without Division of Water Resources approval. Utah allows up to two covered containers of no more than 100 gallons each without registration; larger registered systems can store up to 2,500 gallons. Check current state and local rules, including permitted uses, setbacks, overflow, and plumbing requirements, before installing a system.

It is the size of a US standard steel drum, the same 55 gallon container the petrochemical industry standardized on in the early 1900s. Recycled food-grade drums (originally used for juice, syrup, or olives) are plentiful, cheap, food-safe, and roughly the right footprint to tuck under a downspout. A 55 gallon barrel weighs about 459 lb when full, which is the upper edge of what residential structures and stands can support without engineering. Larger 100 to 300 gallon polytanks are common at the next size up, but they need a reinforced base and look much more industrial in a yard.