How much water from 1 inch of rain on a 1,000 sq ft roof?
One inch of rain on a 1,000 sq ft roof is 623 gallons of water. An asphalt-shingle roof sheds about 90% of it, and a first-flush diverter takes about 10 gallons, so you could collect about 551 gallons; you would need 11 barrels of 55 gal to store it all.
How to work it out
- 1,000 sq ft × 1 in × 0.623 gal = 623 gal
- × 0.90 (asphalt shingles) = 561 gal − 10 gal first flush (10 gal per 1,000 sq ft) = 551 gal
- 551 gal ÷ 55 gal = 11 barrels
This is the benchmark number for rainwater harvesting: 0.623 gallons per square foot per inch. Most houses split the roof between several downspouts, so a single barrel usually sees only a quarter to half of this. Count the roof area that actually drains to the downspout you are using.
Adjust for your project
More options: First flush · Storms · Use
- Likely collected
- 551 gal
- Barrels to hold it all
- 11 barrels
- Rain needed to fill one barrel
- 0.12 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.
Water caught from 1 in of rain on 1,000 sq ft, by roof type
| Roof | Likely collected | Barrels to hold it all |
|---|---|---|
| Metal | 582 gal | 11 barrels |
| Asphalt shingles | 551 gal | 11 barrels |
| Clay or concrete tile | 520 gal | 10 barrels |
| Flat roof with gravel | 489 gal | 9 barrels |
| Green (planted) roof | 302 gal | 6 barrels |
What to know about 1 inch of rain on a 1,000 sq ft roof
Six hundred gallons from one ordinary rainy day surprises most people. The catch is that it arrives in a few hours, and a barrel only holds 55 of it.
- Per downspout, not per house. With four downspouts, each one typically drains around 250 sq ft. An inch of rain then sends about 156 gallons down each spout, roughly 138 after the roof and first-flush losses: two full barrels and plenty of overflow.
- It fills in the first tenth of an inch. If the whole 1,000 sq ft feeds one 55 gal barrel, it is full after about 0.12 in of rain, counting the 10 gallons the first-flush diverter takes. Everything after that has to go somewhere, so the overflow hose or diverter matters more than the barrel.
- Roof type moves it a little. A metal roof sheds about 95% of the rain (roughly 582 gallons caught here), a gravel-topped flat roof nearer 80%. The table below compares them.
- Weight adds up. Each full 55 gal barrel weighs over 450 lb. Two linked barrels on a stand is close to half a ton on a small footprint; build the stand on level, compacted gravel.
- Using the surplus. The 551 gallons you could catch would more than fill a small garden pond — an 8 × 6 ft pond holds around 375 gallons (see What size liner for an 8×6 pond, 2 ft deep).
For a whole year of rain on a bigger roof, see How much rainwater a 2,000 sq ft roof collects in a year. For a single small roof that suits one barrel well, see How much rainwater from a 10×12 shed 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) - 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)
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 many gallons from half an inch of rain on a 1,000 sq ft roof?
About 312 gallons lands on it, or 271 gallons after a shingle roof’s losses and a 10 gallon first flush.
How much rain fills a 55 gallon barrel from a 1,000 sq ft roof?
Only about 0.12 inches of rain on a shingle roof, including the 10 gallons a first-flush diverter takes.
How much water from 1 inch of rain on a 1,500 sq ft roof?
About 935 gallons; roughly 827 gallons could be collected from a shingle roof after the first flush.
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%.