Rain Barrel Calculator: How Much Water Your Roof Can Collect

The math behind the gallons

An inch of rain falling on one square foot of roof is 144 cubic inches of water. A US gallon holds 231 cubic inches, so every square foot sheds 0.623 gallons per inch of rain. Metric is tidier still: 1 millimeter of rain on 1 square meter is exactly 1 liter.

gallons = roof area (sq ft) × rainfall (inches) × 0.623 × efficiency

The same constant sits behind the USGS Water Science School's rainfall tables, which put one inch of rain on an acre at about 27,154 gallons.

Efficiency covers the water that never reaches the barrel. Gutters spill over in a downpour, wind carries rain past the edge, some of it wets the shingles and evaporates, and a first-flush diverter (if you fit one) dumps the dirtiest first gallons on purpose. The calculator starts at 80 percent. A smooth metal roof with clean gutters can beat that; a gravel roof or gutters clogged with leaves will fall short. Barrels are rounded up, because a storm that fills five and a half barrels still needs a sixth place for the water to go.

Worked example: 600 square feet and a 1-inch storm

Leave the calculator on its starting values: 600 square feet of roof draining to the downspout, 1 inch of rain, 80 percent efficiency and 50-gallon barrels.

• 600 × 1 × 0.623 = 373.8 gallons land on that part of the roof.
• At 80 percent, 299 gallons (1,132 liters) reach the barrel.
• 299 ÷ 50 = 5.98, so the result reads 6 barrels.

That surprises most people. One ordinary storm on a modest section of roof fills a single barrel six times over. Even a light quarter-inch shower on the same roof sends about 75 gallons, enough for one barrel and half of a second. A bigger house shows the same pattern: 1,500 square feet in half an inch of rain gives about 374 gallons.

To find your own roof area, measure the footprint from the ground, not the sloped surface. Rain falls straight down, so a steep roof catches no more than a flat one over the same footprint. Most houses split the roof between two and four downspouts; walk around during a rain and see which spout runs hardest.

Turning gallons into a plan

Because a single storm overfills a barrel, the question that matters is less "how big a barrel" and more "where does the overflow go." Three common answers:

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1. A diverter in the downspout. A downspout diverter fills the barrel and, once it is full, sends water back down the spout as before.
2. A second barrel linked by a short hose near the top, so the first one spills into the next.
3. An overflow hose aimed at a rain garden or lawn well away from the foundation.

Then match storage to what you actually water. Gardens need roughly an inch of water a week in summer, and one 50-gallon barrel delivers that inch to about 80 square feet of bed: two 4-by-10-foot beds. A full barrel weighs more than 400 pounds, so set it on level, solid ground or concrete blocks. Raising it also gives the spigot more pressure.

Rooftop water is for plants, not people. The EPA's Soak Up the Rain program warns that rain picks up bacteria from birds and chemicals from roofing as it runs off. Keep a tight lid and a fine screen on every opening so mosquitoes cannot breed inside.

Before you buy anything, ask your city or water utility. Many run spring rain barrel sales or rebates, and some hand barrels out free at workshops. Renters can usually set a freestanding barrel under a spout without any permanent change; cutting a downspout for a diverter alters the building, so get the landlord's yes in writing first. The same habit of putting requests in writing helps with the leaks and water bills covered in our guide to low-waste living on a tight budget.

These picks come from maker specifications, published reviews and owner feedback; we have not put them through hands-on trials of our own.

If the barrel water is headed for vegetables, our compost calculator works out how much compost the same beds need, and compost-rich soil holds that water longer between storms.

Questions

How many gallons of water come off a roof per inch of rain?

Each square foot of roof sheds about 0.623 gallons for every inch of rain, before losses. A 1,000-square-foot roof produces about 623 gallons in a 1-inch storm, or roughly 500 gallons if about 80 percent of it reaches the downspout.

How many rain barrels do I need?

Divide the gallons from a typical storm by the size of your barrel and round up. For most homes a single storm overfills one barrel several times, so plan storage around how much water you use between rains, and send the rest to an overflow hose, a linked barrel or a downspout diverter.

Is collecting rainwater from my roof legal?

Rooftop collection in rain barrels is allowed across most of the United States, but a few states set limits. Colorado, for example, allows up to two barrels with a combined 110 gallons per household, for outdoor use on the same property, according to Colorado State University Extension. Check with your state water agency or local extension office.

Can you drink water from a rain barrel?

Not without proper treatment. Water running off a roof can carry bacteria from bird droppings and chemicals from roofing materials, so rain barrel water is meant for gardens, lawns and outdoor cleaning.

How do I keep mosquitoes out of a rain barrel?

Use a barrel with a sealed lid and put fine mesh screen over the inlet and any overflow opening. Check the screens after storms and use the water regularly so it does not sit for long stretches.