How much rainwater from a 10×12 shed roof?
A 10 × 12 ft shed roof (120 square feet) sheds about 74.8 gallons per inch of rain, roughly 66.1 gallons after a shingle roof’s losses and a small first flush. A 55 gallon barrel fills with about 0.83 inches of rain.
How to work it out
- 120 sq ft × 1 in × 0.623 gal = 74.8 gal
- × 0.90 (asphalt shingles) = 67.3 gal − 1.2 gal first flush (10 gal per 1,000 sq ft) = 66.1 gal
- 66.1 gal ÷ 55 gal = 2 barrels
Sheds and greenhouses suit a single barrel: the roof is small enough that the barrel doesn’t overflow in every storm, and the water is right where the garden is. Most sheds need a gutter added on at least one side first.
Adjust for your project
More options: First flush · Storms · Use
- Likely collected
- 66.1 gal
- Barrels to hold it all
- 2 barrels
- Rain needed to fill one barrel
- 0.83 in
- A single barrel fills in a light rain — plan an overflow that carries water away from the foundation.
- Rules on collecting rainwater differ by state and country, and some places regulate volume or use. Check yours before installing a large system.
Rain needed to fill one barrel from a 120 sq ft shed roof, by barrel size
| Barrel size | Rain needed to fill one barrel | Barrels to hold it all |
|---|---|---|
| 30 gal | 0.46 in | 3 barrels |
| 50 gal | 0.76 in | 2 barrels |
| 55 gal | 0.83 in | 2 barrels |
| 65 gal | 0.98 in | 2 barrels |
| 100 gal | 1.5 in | 1 barrel |
What to know about collecting from a 10×12 shed
- Measure the overhang too. The footprint is the roof, not the walls. A 10 × 12 shed with 6 in eaves all round covers 11 × 13 ft, or 143 sq ft, about a fifth more water than the wall size suggests.
- Gable or lean-to. A gable (peaked) roof sheds half its water each way, so one gutter catches only about 60 sq ft: roughly 37 gallons per inch, and it takes about 1.65 in of rain to fill a 55 gal barrel. A single-slope lean-to roof sends everything to one gutter.
- Metal or shingle. Many sheds have metal roofs, which shed about 95% of the rain against about 90% for shingles. On 120 sq ft the difference is only a few gallons an inch, but metal also stays cleaner.
- A barrel that actually empties. Because it takes most of an inch to fill, a shed barrel overflows far less often than one on a house downspout, and you will use it up between storms if it feeds a greenhouse or a few beds nearby.
- Screen it. Shed gutters collect leaves and seeds; a screened inlet keeps mosquitoes and debris out, as Clemson’s rain barrel guide recommends.
A shed barrel is also a handy way to top up a small pond without tap water: see What size liner for an 8×6 pond, 2 ft deep. For a house roof, compare How much water from 1 inch of rain on a 1,000 sq ft roof.
Different size? Use the full Rain Barrel Calculator.
What you’ll need
- Rain barrel (50–65 gal)Mid-range price
A lidded barrel with a screened inlet, spigot and overflow. Linkable models make it easy to add a second one later.
See options on Amazon (opens in a new tab) - Downspout diverterBudget price
Fits into the downspout and sends water to the barrel until it is full, then back down the spout.
See options on Amazon (opens in a new tab) - Soaker hoseBudget price
Weeps water slowly along its length — good for beds and rows, and low enough pressure to run from a rain barrel.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much rain fills a 50 gallon barrel from a 10×12 shed roof?
About 0.76 inches of rain, assuming a shingle roof and a small first-flush diverter.
How much rainwater from an 8×10 shed roof per inch of rain?
About 49.9 gallons per inch, roughly 44.1 gallons after losses.
How much rainwater from a 12×16 shed roof per inch of rain?
About 120 gallons per inch; a 55 gal barrel fills with about 0.53 inches.
How much rainwater from a 10×12 metal shed roof per inch?
About 69.9 gallons reaches the barrel: metal sheds about 95% of the 74.8 gallons that lands on it.
Other common sizes
Sources
- Clemson Cooperative Extension (HGIC) — Designing for Rain Barrel Success (opens in a new tab) — Roof area × 0.623 = gallons from a 1 in rain; screen the inlet; emergency overflow away from the foundation; barrels raised 12–36 in.
- Clemson Cooperative Extension (HGIC) — Best Practices for Application of Harvested Rainwater on Edibles (opens in a new tab) — Water the soil, not the leaves; not on harvest day; wash produce in potable water; first-flush diversion.
- Texas Water Development Board — The Texas Manual on Rainwater Harvesting (3rd ed.) (opens in a new tab) — First flush: divert a minimum of 10 gallons per 1,000 sq ft of collection surface (1–2 gal per 100 sq ft); about 0.62 gal per sq ft per inch in theory; example uses 85% collection efficiency.
- New Mexico Office of the State Engineer — Roof-Reliant Landscaping, Ch. 7: Water Collection (opens in a new tab) — Runoff coefficients: metal 0.95, asphalt 0.90, concrete 0.90, tar and gravel 0.80–0.85.
- Texas A&M AgriLife Extension — Rainwater Harvesting: System Planning (B-6240) (opens in a new tab) — Roof runoff coefficients 0.75–0.95 (smooth surfaces high, rough low); monthly supply vs demand planning.
- Minnesota Stormwater Manual — Overview for green roofs (opens in a new tab) — Green roofs with 3–6 in of media typically cut annual runoff by 50–70%.