Start with one downspout
A rain barrel collects only what its downspout delivers, which is the part of the roof draining to that gutter run. Estimate that area from above: for a simple gable roof with a downspout at each end of one gutter, each downspout typically drains roughly half of that side of the roof.
gallons per storm = roof area to downspout (sq ft) × rainfall (in) × 0.623 × efficiency
barrels = gallons ÷ barrel size, rounded up
Worked example
A downspout drains 500 sq ft of roof. Assume 80 percent efficiency.
| Storm | Gallons collected | 55-gallon barrels to hold it |
|---|---|---|
| 0.5 in of rain | about 125 gal | 3 |
| 1 in of rain | about 249 gal (944 L) | 5 |
Even a half-inch rain fills more than two standard barrels from a modest piece of roof. Run your own numbers with the rainwater harvesting calculator.
You do not need to catch every drop
Storage is only useful if you empty it between storms. A barrel that sits full overflows and catches nothing more. Size for the water you will actually use for the garden in a week or two, then let the rest overflow safely.
| Goal | Practical starting point |
|---|---|
| Watering cans for pots and a few beds | 1 or 2 barrels on one downspout |
| Vegetable garden with several raised beds | 2 to 4 linked barrels |
| Larger garden or lawn areas | A larger tank, sized by your weekly use |
To estimate weekly garden use, a common rule of thumb is about an inch of water a week for vegetable beds in dry weather. One inch over 100 sq ft of beds is about 62 gallons. Sizing your beds? See the raised bed soil calculator.
Common container sizes
| Container | Typical capacity |
|---|---|
| Standard rain barrel | about 50 to 60 gal (190 to 230 L) |
| Slimline wall tank | varies, often a few hundred liters |
| Intermediate bulk container (tote) | commonly 275 or 330 gal |
| Plastic or metal tank | hundreds to thousands of gallons |
Used food-grade containers are popular for rain barrels. Avoid any that held chemicals, and check the capacity marked on the container.
Linking barrels
- Side by side: connect barrels near the bottom with a short hose or fitting so they fill and drain together.
- In series: run the overflow of the first barrel into the second. Simple, but the first barrel collects most of the debris.
- Keep all barrels at the same height on a firm, level stand. Raising them helps water flow from the spigot.
Plan the overflow
When barrels are full, the extra water has to go somewhere. Send overflow through a hose or pipe at least several feet from the foundation, into a rain garden or onto a lawn that drains well. Never let it pool against the house or flow onto a neighbor's property.
Safety and good practice
- A full 55-gallon barrel weighs more than 450 lb. Use a solid base that cannot sink or tip.
- Fit tight lids and fine screens to keep out mosquitoes, debris and children. Never leave an open barrel.
- Roof water is not drinking water. Use it for plants and outdoor cleaning.
- Rules vary: most US states allow rain barrels, but a few restrict collection. Colorado, for example, limits households to two barrels with a combined capacity of 110 gallons. Check local rules before buying.
Winterize barrels before the first freeze
Water expands as it freezes and can split plastic barrels and break spigots. Before the first hard freeze:
- Drain each barrel completely and use the water on beds or trees.
- Disconnect the barrels and put the downspout extension back so roof water flows away from the foundation.
- Rinse barrels, open the spigots and store them upside down or under cover.
- Clean the gutters after the leaves fall so they flow freely in spring.
In mild climates without hard freezes, barrels can stay in use all year. Keep screens clean so winter rains are not wasted.
Frequently asked questions
How fast does a rain barrel fill?
Fast. About 125 gallons runs off a 500 sq ft section of roof in a half-inch rain at 80 percent efficiency, which fills more than two 55-gallon barrels.
Can you have too many rain barrels?
Yes, if you never use the water. Storage only helps if you empty it between storms, so size barrels to your actual watering needs.
Should I empty rain barrels for winter?
Where it freezes, yes. Drain, disconnect and store them so ice does not crack them, and redirect the downspout away from the house.
Do the math for your own project.
Open the rainwater harvesting calculator