Key takeaways
- Gallons = roof area × inches of rain × 0.623 × runoff coefficient.
- Nebraska Extension's example gives about 750 gallons from 1,200 sq ft in a 1-inch rain, before any loss to the roof surface.
- A 55-gallon barrel on 300 sq ft of roof fills after about a third of an inch of rain, so plan the overflow.
Enter the roof area that drains to one downspout, measured level, a rainfall depth in inches, and your barrel size. The tool gives the gallons that run off, how many barrels that fills, and how much rain it takes before a barrel overflows.
How the calculator works
- Gallons = roof area × rainfall in inches × 0.623 × runoff coefficient
- Barrels = gallons ÷ barrel size, rounded up
- Rain that fills one barrel = barrel size ÷ (roof area × 0.623 × coefficient)
The 0.623 converts inches of rain on a square foot into gallons. One inch on 1 square foot is 144 cubic inches, and a gallon is 231 cubic inches, so 144 ÷ 231 = 0.623. Nebraska Extension's rain barrel guide and the University of Arizona's Harvesting Rainwater for Landscape Use both use it.
The runoff coefficient is the share of the rain that actually leaves the roof. Some wets the surface, splashes or evaporates. The default is 0.9, the low end of the University of Arizona's range for metal, gravel, asphalt shingle, fiberglass and mineral paper roofs, which runs from 0.90 to 0.95. Set it to 1 to match Nebraska Extension's figures, which don't use a coefficient.
Use the level area, not the sloped area
Rain falls straight down, so what counts is the area the roof covers seen from above. The University of Arizona guide says to measure a sloped roof by the area it covers. That's the same footprint the roof area calculator starts from, before it applies the pitch.
Only count the part of the roof that drains to the downspout feeding your barrel. Nebraska Extension's example assumes a quarter of the roof drains to each downspout. Look at where your gutters actually slope.
Worked example
Nebraska Extension's example: a 1,200-square-foot roof in a 1-inch rain.
- 1,200 × 1 × 0.623 = 747.6, or about 750 gallons (coefficient 1)
- With the 0.9 default: 747.6 × 0.9 = 673 gallons
Now one downspout draining a quarter of that roof, 300 square feet:
- 300 × 1 × 0.623 = 186.9 gallons, which the guide rounds to about 185
- With the 0.9 default: 186.9 × 0.9 = 168 gallons
- Barrels: 168 ÷ 55 = 3.06, so 4 barrels to hold it all
- One barrel fills after 55 ÷ (300 × 0.623 × 0.9) = 55 ÷ 168.2 = 0.33 inches of rain
So a single 55-gallon barrel on that downspout overflows in about a third of an inch of rain. Nebraska Extension makes the same point: in a 1-inch rain, a typical 55-gallon container on that downspout will overflow.
Runoff by rainfall
For 300 square feet draining to one downspout, coefficient 0.9, 55-gallon barrels:
| Rainfall | Runoff | Barrels to hold it |
|---|---|---|
| 0.25 in | 42 gal | 1 |
| 0.5 in | 84 gal | 2 |
| 1 in | 168 gal | 4 |
| 2 in | 336 gal | 7 |
For a year's total, use your annual rainfall. The University of Arizona's Maricopa County guide gives that area's average as 7 to 8 inches a year. A 1,000-square-foot roof there at 7 inches gives 1,000 × 7 × 0.623 × 0.9 = 3,925 gallons a year. Your barrels only keep what fits in them each storm, so storage size, not the annual total, sets how much you actually save.
Plan the overflow
Every barrel will fill, so decide where the extra goes before it rains. Nebraska Extension says to send overflow:
- into more barrels, or to a planted area, ideally a rain garden
- not onto a driveway, sidewalk or other hard surface, where it can reach a storm drain
- not toward your foundation, a neighbor's property, or any part of a septic system
The gutter and downspout calculator explains the IRC's grading figure: ground should fall at least 6 inches within 10 feet of the foundation. Overflow should leave the house the same way.
Setting up a barrel
- Weight. Nebraska Extension notes that a full 55-gallon barrel weighs over 450 pounds, so set it on a flat, firm base.
- Height. A sturdy platform lifts the spigot high enough for a watering can or hose.
- Anchoring. The guide also says to anchor barrels to the building or another secure object.
Rules on collecting rainwater vary, so check with your city or state before a large setup.
Common mistakes
- Using the sloped roof area. It overstates the runoff on steep roofs. Use the level footprint.
- Counting the whole roof for one barrel. Only the part draining to that downspout fills it.
- Sizing for the year, not the storm. A small barrel fills in one modest rain, however much falls in a year.
- Leaving out the overflow. Without a planned route, the extra water ends up at your foundation.
- Forgetting the weight. A full barrel on soft ground can tip.
Frequently asked questions
How many gallons of water come off a roof in 1 inch of rain?
Multiply the level roof area by 0.623. A 1,200-square-foot roof gives about 750 gallons, or about 673 if you allow for 10 percent loss on the roof.
How do I calculate rainwater runoff from my roof?
Multiply the level area in square feet by the rainfall in inches, then by 0.623 for gallons. Multiply by a runoff coefficient, such as 0.9, to allow for what the roof holds back.
How fast will a rain barrel fill?
A 55-gallon barrel on 300 square feet of roof fills after about a third of an inch of rain. A bigger area fills it faster.
How many rain barrels do I need?
Divide the runoff from the storm you want to capture by your barrel's size, and round up. Holding a full inch from 300 square feet takes four 55-gallon barrels.
Why is 0.623 used?
An inch of rain on a square foot is 144 cubic inches of water. A gallon is 231 cubic inches, and 144 ÷ 231 = 0.623.
What roof area should I use for gutters and rain barrels?
The level footprint for both. The gutter calculator uses the same figure to size gutters and downspouts.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- University of Nebraska–Lincoln Extension: Rainwater harvesting with rain barrels (G2220)
- University of Arizona Cooperative Extension: Harvesting Rainwater for Landscape Use (AZ1344)
- University of Arizona Cooperative Extension: Using Rainwater in Urban Landscapes, Maricopa County (AZ1566)
- 2021 International Residential Code, Chapter 4, Section R401.3