How much rainwater can a 2,000 sq ft roof collect in a year?
With 40 in of rain a year, a 2,000 sq ft roof sheds about 49,870 gallons. From an asphalt-shingle roof, with a first-flush diverter emptying at each of about 40 storms, roughly 44,083 gallons could be captured if you had somewhere to store it.
How to work it out
- 2,000 sq ft × 40 in × 0.623 gal = 49,870 gal
- × 0.90 (asphalt shingles) = 44,883 gal − 800 gal first flush (10 gal per 1,000 sq ft × 40 storms) = 44,083 gal
- 44,083 gal ÷ 55 gal = 802 barrels
Annual totals are eye-opening, but storage is the limit, not supply. Rain arrives in bursts, so a few barrels refilled all season often do more practical good than one large tank that sits full between dry spells. Put in your own annual rainfall and rainy days; both vary hugely by region.
Adjust for your project
More options: First flush · Storms · Use
- Likely collected
- 44,083 gal
- Barrels to hold it all
- 802 barrels
- Rain needed to fill one barrel
- 0.07 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.
A year of rain (40 in, 40 storms) by roof area
| Roof area | Rain falling on that roof | Likely collected |
|---|---|---|
| 1000 sq ft | 24,935 gal | 22,042 gal |
| 1500 sq ft | 37,403 gal | 33,062 gal |
| 2000 sq ft | 49,870 gal | 44,083 gal |
| 2500 sq ft | 62,338 gal | 55,104 gal |
| 3000 sq ft | 74,805 gal | 66,125 gal |
What to know about a year of rain on a 2,000 sq ft roof
Nobody stores forty thousand gallons. The useful question is how much of it you can use, and when.
- Think in storms, not years. A single 1 in storm on this roof sheds about 1,247 gallons, about 1,102 after the roof and first-flush losses. A 1,000–1,500 gallon tank can catch one storm; most of the year’s total runs off whatever you build.
- The first flush adds up. At 10 gallons per 1,000 sq ft per storm, 40 storms send about 800 gallons to the diverter over a year. That is the price of cleaner water in the tank, and a small one next to the total.
- Match storage to demand. A 100 sq ft vegetable bed that needs about an inch of water a week uses roughly 62 gallons a week, or about 1,250 gallons over a 20-week season. That is about one storm’s worth from this roof, spread over months, which is why a tank refilled by summer storms goes further than its size suggests. Enter a month’s rain and a month’s use under More options to size it properly.
- Tanks are heavy. Water weighs about 8.3 lb per gallon, so a 1,000 gallon tank is over four tons full. It needs a level, compacted base, and it needs to be well clear of the house foundation.
- Rainfall is seasonal. Forty inches a year rarely falls evenly; in many places the driest months are exactly when gardens need water most. Check the monthly averages for your area and size storage for the dry stretch.
Per storm on a smaller roof, see How much water from 1 inch of rain on a 1,000 sq ft roof. In metric units, see How much rainwater from 25 mm on a 100 m² 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) - Rain barrel linking kitBudget price
Hoses and fittings to join barrels side by side, so overflow from the first fills the next instead of running off.
See options on Amazon (opens in a new tab) - Rain gaugeBudget price
Tells you how much of this week’s inch the sky already delivered. Cheap and surprisingly useful.
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 rainwater from a 2,000 sq ft roof with 30 inches of rain a year?
About 37,403 gallons falls on the roof; roughly 32,862 gallons could be caught from shingles over about 40 storms.
How much rainwater from a 2,000 sq ft roof with 50 inches of rain a year?
About 62,338 gallons, or 55,304 gallons after roof and first-flush losses.
How much water from a 1 inch storm on a 2,000 sq ft roof?
About 1,247 gallons, roughly 1,102 gallons after losses; you would need 21 barrels of 55 gal to hold one storm.
How much rainwater does a 2,000 sq ft metal roof collect in a year?
About 46,577 gallons from 40 in of rain: metal sheds about 95% of what lands on it, the most of any common roof.
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%.