Understanding your result
The headline is the water collected from the rain you entered. The stats give the collection per inch, the barrels, the overflow and, if you entered it, the yearly total. The breakdown shows the runoff per inch, the efficiency, the barrels, the rain that fills one barrel and the overflow.
The number to remember is the rain that fills one barrel. It is usually a fraction of an inch, which means the overflow is the main event: it must be piped away from the foundation like the downspout itself, which the downspout extensions calculator covers. The roof area draining to each downspout is the same figure the downspout calculator uses.
Watering the garden with it is a question for the garden: the companion site The Woven Garden covers irrigation planning, and the footer links it.
How we calculate this
The constant 0.623 is exact geometry: an inch of water over a square foot is 144 cubic inches, and a US gallon is 231 cubic inches. In metric the arithmetic is simpler still, a litre per square metre per millimetre. The area is the plan area, which the roof area calculator reports as the footprint.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Runoff per sq ft per inch | 0.623 US gal | 144 cu in ÷ 231 cu in per gallon |
| Efficiency | 80% | Rainwater harvesting guides (75 to 90% for metal and shingles) |
| Barrel | 55 gal | The common barrel size |
| Roof area | 500 sq ft | A quarter of a modest house draining to one downspout |
Assumptions last reviewed October 7, 2026.
The calculator does not model evaporation from the barrel, water use between storms or local rules on harvesting, which some states and cities regulate. The gutter sizing guide explains the plan-area convention, and the gutter and downspout sizing table shows how the same runoff sizes the gutter that feeds the barrel.
Two worked examples
One downspout and one barrel
500 square feet of roof, 1 inch of rain, 80 percent, a 55-gallon barrel, 38 inches of rain a year.
- Runoff per inch: 500 × 0.623 = 312 gal; collected: 312 × 0.8 = 249 gal
- Barrels: 249 ÷ 55 = 4.5, so 5
- One barrel fills in 0.22 in of rain; overflow: 194 gal
- Yearly: 9,470 gal
A single barrel catches a fifth of a one-inch storm.
A larger roof face and a 275-gallon tote
1,200 square feet, ½ inch of rain, 75 percent, a 275-gallon tote.
- Runoff per inch: 748 gal; collected: 748 × 0.5 × 0.75 = 280 gal
- One tote fills in 0.49 in of rain; overflow: 5 gal
A tote almost holds a half-inch storm from this roof; anything bigger overflows.
Where to find your inputs
Roof area. The plan area of the roof faces draining to the downspout.
Rainfall. For a storm, a forecast or a rain gauge; for a year, the NOAA climate normals for your town.
Barrel size. On the barrel: 55 gallons is standard, IBC totes are 275 or 330.
Common mistakes
- Using the sloped area. Rain falls vertically. Use the plan area.
- Expecting a barrel to catch a storm. It fills in a fraction of an inch.
- Overflow at the foundation. Pipe it away.
- Ignoring the first flush. The dirtiest water comes first.
- Drinking it. Not without treatment.
- Leaving it open. Screen it against mosquitoes.
Questions people ask
- How much water comes off a roof per inch of rain?
- 0.623 US gallons per square foot of plan roof area per inch of rain, because an inch of water over 144 square inches is 144 cubic inches. A 1,000 square foot roof sheds 623 gallons per inch; with 80 percent collected, about 500. In metric, 1 mm of rain on 1 m² is 1 litre.
- How fast does a rain barrel fill?
- Divide the barrel by the runoff per inch. A 500 square foot roof face collecting at 80 percent yields 249 gallons per inch, so a 55-gallon barrel fills in 0.22 inches of rain, a light shower. Most storms overflow a single barrel.
- How much water can I collect in a year?
- Annual rainfall times the runoff per inch: a 500 square foot roof face in a 38-inch climate yields about 9,470 gallons at 80 percent. You only keep what your barrels or tank hold between uses.
- Should I use the roof area or the sloped area?
- The plan area, the horizontal footprint of the roof face: rain falls vertically, so a steep roof catches no more than a flat one of the same footprint, apart from wind.
- What collection efficiency should I use?
- 75 to 90 percent for metal and shingles, lower for rough or absorbent roofs and where a first-flush diverter discards the first water of each storm. 80 percent is a fair middle.
- Is rain barrel water safe to drink?
- No, not without treatment. Roof water picks up debris, bird droppings and compounds from roofing materials. It is for gardens, washing and similar uses.