How the calculation works

Rain that lands on a roof runs into the gutters, so the water you can catch is the roof area multiplied by the depth of rain, less what is lost on the way.

Roof area: use the footprint, which is the length times the width of the roof from eave to eave. The Texas Water Development Board's rainwater manual says to use the footprint whatever the pitch of the roof. If only part of the roof drains to your downspout, count only that part.
US units: 1 inch of rain on 1 square foot is 144 cubic inches of water, and a US gallon is 231 cubic inches, so gallons = square feet × inches of rain × 0.6234. The Texas Water Development Board rounds this to 0.62 gallons per square foot per inch.
Metric: 1 mm of rain on 1 m² is exactly 1 litre, so litres = square metres × millimetres of rain.
Losses: the same manual says that in practice some water is lost to the first flush, evaporation, splash, gutters overshooting in hard rain and possibly leaks, and that most installers assume an efficiency of 75 to 90 percent. The calculator starts at 85%, the figure used in the manual's own worked example. Enter 100 to see the theoretical maximum.
Containers: the water is divided by each container size, and the number needed is rounded up to whole containers. Overflow is whatever is left once your storage is full.

Worked example

A downspout drains a 20 × 15 ft section of roof, which is 300 square feet. A 1-inch storm drops 300 × 1 × 0.6234 = 187 gallons on it, and at 85% you collect about 159 gallons (602 litres). That is 2.9 barrels' worth, so it takes three 55-gallon barrels to hold it all. With one barrel, 104 gallons overflow; with two (110 gallons), 49 gallons overflow. On this roof, about 0.35 inch of rain fills one 55-gallon barrel.

In metric, a 6 × 5 m roof is 30 m². A 25 mm storm drops 750 litres on it, and at 85% you collect 637.5 litres. That fills a 210 L water butt about three times over (you would need four to hold it all), or fits in one 1,000 L IBC. With a single 210 L butt, 427.5 litres overflow, and 8.2 mm of rain is enough to fill it.

As a check, Rutgers Cooperative Extension estimates that an 800 sq ft roof drains about 600 gallons in a 1.25-inch storm. The formula gives 623 gallons before losses, or 530 gallons at 85%.

Where to find your rainfall

For one storm, use a rain gauge or a weather report. For a typical month or year, look up the long-term averages for the weather station nearest you.

US: NOAA climate normals
Canada: Environment and Climate Change Canada climate normals
UK: Met Office long-term averages
Australia: Bureau of Meteorology climate data

A month's or a year's rain falls over many storms. Water you use in between makes room for more, so the monthly and yearly overflow figures are the most that could overflow if you used none. Colorado State University Extension notes that a typical rain barrel user there can only expect their barrels to fill completely about 10 to 15 times in a growing season.

Setting up a rain barrel safely

Send the overflow away from the house. Rutgers Cooperative Extension suggests attaching a hose to the overflow and leading it to a lawn, garden or bed where it can soak in, at least 6 feet from the foundation if there is a basement and at least 2 feet if there is a crawl space. The Texas Water Development Board adds that tank overflow should be routed so it does not affect the foundation of the tank or any other structure, and should not run into a septic system drainfield.
Keep mosquitoes out. Rutgers says to keep a screen on every opening at all times, and to keep a hose on the overflow because mosquitoes can get in that way too. Colorado State University Extension suggests a tight-fitting lid or mesh, emptying the barrel completely at least once a month, and disconnecting it from the downspout if you will be away for more than a week. The RHS adds that a close-fitting lid keeps out plant-disease spores and discourages algae.
Do not drink it. Rutgers and Colorado State both say rain barrel water is not for drinking. Rutgers also advises washing any vegetables you water with it in drinking water before eating them, and not collecting rain for three months after a moss killer has been used on the roof.
Consider a first-flush diverter. The first rain off a roof washes down dust, pollen, bird droppings and other debris. A first-flush diverter sends this water away before the rest flows to the barrel. The Texas Water Development Board says opinions vary on how much to divert; the range it recommends is 1 to 2 gallons for every 100 square feet of roof, and the calculator shows that amount for your roof.
Plan for the weight and the winter. Rutgers says a full 55-gallon barrel weighs about 460 pounds. It suggests raising the barrel on a platform, which helps the water flow out, on level ground so the barrel cannot tip over when full. Drain it and disconnect it from the downspout for winter so water does not freeze inside.
Use it on established plants. The RHS suggests using stored rainwater on established plants but not on seedlings, because it may carry damping-off diseases.

Check the rules where you live

In Colorado, the Division of Water Resources says that under Section 37-96.5-103 of the Colorado Revised Statutes a home may collect rain from its roof, without a permit, in at most two rain barrels with a combined capacity of 110 gallons, with sealable lids, and use the water outdoors on the property where it was collected. This covers single-family homes and buildings with four or fewer units, and the water cannot be used for drinking or indoors. Homes that have, or could qualify for, certain residential well permits can apply for a separate permit to collect more. This page does not cover the rules anywhere else, so check what applies where you live before you buy.

Common questions

How much water does 1 inch of rain on a roof make?: Every 1,000 sq ft of roof footprint sheds 623 US gallons (2,360 litres) in 1 inch of rain before losses, or about 530 gallons at 85% collection efficiency. In metric, 1 mm of rain on 1 m² is 1 litre, so 10 mm on a 100 m² roof is 1,000 litres, or 850 litres at 85%.
Do I measure the sloped roof or the ground it covers?: The ground it covers. The Texas Water Development Board's rainwater manual says the collection area is the roof's footprint, length times width from eave to eave, whatever the pitch. A 40 × 30 ft house has a 1,200 sq ft footprint. Count only the part of the roof that drains to the downspout feeding your barrel.
How many rain barrels do I need?: It depends on the roof area and the rain. With 300 sq ft of roof draining to one downspout and 85% efficiency, about 0.35 inch of rain fills a 55-gallon barrel, and a 1-inch storm makes about 159 gallons, which takes three barrels to hold. Whatever does not fit overflows, so lead the overflow away from the house.
Can I drink water from a rain barrel?: No. Rutgers Cooperative Extension says rain barrel water should not be used as a drinking water source, and Colorado State University Extension says untreated rainwater from roofs is not safe to drink. Systems built to supply drinking water need treatment equipment, and the Texas Water Development Board says such water should be tested, at a minimum, for pathogens.
Are there rules about rain barrels?: There can be. In Colorado, for example, the Division of Water Resources says a home can collect rain from its roof in two rain barrels with a combined capacity of up to 110 gallons without a permit, and use the water outdoors on the same property. Homes that have, or could qualify for, certain residential well permits can apply for a permit to collect more. This page does not cover the rules anywhere else, so check what applies where you live before you buy.

Sources

Last checked and updated: November 1, 2026.