Solve backwards
How much roof fills my rain barrel in an inch of rain?
- 55 galThe example worked below
- Rain Barrel and Rainwater Collection CalculatorEvery figure is its own
You have
How much rain barrel?
That is 55 gal(208.2 L). Worked by the Rain Barrel and Rainwater Collection Calculator at the settings below.
The settings it uses
- Rainfall Amount
- 1 in
What it will do
88.3 sq ft of roof
8.2 m²
One roof footprint. Enough for a roof footprint about 9.4 × 9.4 ft.
Check it forwards
- Roof Catchment Area
- 88.3 sq ft
- It asks for
- 55 gal
- You have
- 55 gal
The Rain Barrel and Rainwater Collection Calculator asks for 55 gal at that size.
If you had more, or less
Around your amount
Each worked by the Rain Barrel and Rainwater Collection Calculator at the same settings.
Half
27.5 gal
44.1 sq ft
a roof footprint about 6.6 × 6.6 ft
Yours
55 gal
88.3 sq ft
a roof footprint about 9.4 × 9.4 ft
Half as much again
82.5 gal
132 sq ft
a roof footprint about 11.5 × 11.5 ft
Double
110 gal
177 sq ft
a roof footprint about 13.3 × 13.3 ft
Common amounts
What each amount of rain barrel covers
The sizes it is usually sold in, each worked by the Rain Barrel and Rainwater Collection Calculator at the settings above.
| You have | It covers | As one roof footprint |
|---|---|---|
| 50 gal(189.3 L) | 80.3 sq ft(7.46 m²) | a roof footprint about 9 × 9 ft |
| 55 gal(208.2 L) | 88.3 sq ft(8.2 m²) | a roof footprint about 9.4 × 9.4 ft |
| 100 gal(378.5 L) | 161 sq ft(14.9 m²) | a roof footprint about 12.7 × 12.7 ft |
| 275 gal(1,041 L) | 441 sq ft(41 m²) | a roof footprint about 21 × 21 ft |
Six tenths of a gallon a square foot
The calculator uses the standard harvesting formula: gallons equal the roof footprint in square feet times the rainfall in inches times 0.623. In metric it is simpler still — a millimetre of rain on a square metre is a litre. So an inch of rain on about 88 square feet (8.2 m²) of roof fills a 55 gallon barrel, and 25 mm on 7.9 m² (85 square feet) fills a 200 litre water butt.
Measure the footprint, not the slope
Use the flat plan area of the roof section draining to the gutter and downspout you collect from — rain falls vertically, so a steep roof catches no more than a flat one over the same ground. A barrel usually sits under one downspout, so only the part of the roof draining to that downspout counts, not the whole house, which is why the answer is often a smaller roof than people expect.
Gross catchment, then the overflow
The figure is gross: no runoff coefficient, nothing for what the roof and gutter hold before runoff starts, and nothing for a first-flush diverter or evaporation, so a real barrel collects meaningfully less. The roof also does not know the barrel's size — once it is full the rest leaves by the overflow — and rainfall intensity never enters it, so an undersized gutter can overshoot before the barrel sees any water.
How the answer is worked
Nothing here is a second formula. The Rain Barrel and Rainwater Collection Calculator runs forwards — a size in, a quantity out — and this page grows its own starting roof footprint in proportion until the quantity it asks for is what you have. Where it rounds up to whole containers, the answer is the largest size before the next one would be needed.
Where the calculator adds an allowance for cuts or breakage when you order, the same allowance comes off here, because it is the same calculator: a stack you already own finishes less than its face value. Where it adds none, neither does this page — the settings beside the answer say which.
Rain Barrel and Rainwater Collection Calculator
runs it forwardsEstimate how many gallons or litres of rainwater you can collect from your roof during a rainfall event.
Work out what to buy insteadSolve backwards
Other things you might already have
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Start from what is already in the yard
Paint, sheets, rolls, bags and pieces, each read the other way by the calculator that orders it.
The calculator's own figures
Nothing is estimated twice: the page runs the same calculator you would order with.
The largest size that fits
Where a calculator rounds up to whole containers, the answer stops before the next one.
Both unit systems
Gallons and litres, square feet and square metres, side by side.
Frequently asked questions
- How much roof does 55 gal of rain barrel cover?
- At the Rain Barrel and Rainwater Collection Calculator's own settings, 88.3 sq ft (8.2 m²) — a roof footprint about 9.4 × 9.4 ft. Every figure follows the settings, which you can change in the calculator.
- How much roof does 200 L of rain barrel cover?
- At the Rain Barrel and Rainwater Collection Calculator's own settings, 7.88 m² (84.8 sq ft) — a roof footprint about 2.8 × 2.8 m. Every figure follows the settings, which you can change in the calculator.
- How much water can I collect from 1 inch of rain?
- About 0.623 gallons per square foot of roof footprint, or a litre per square metre per millimetre — roughly 623 gallons from 1,000 square feet in an inch of rain, or 2,540 litres from 100 m² in 25.4 mm. That is gross; a real roof, gutter and first-flush diverter deliver meaningfully less.
- How fast will my rain barrel fill?
- Often in one moderate storm. A 55 gallon (208 L) barrel under a downspout draining 500 square feet (46 m²) of roof fills in under a fifth of an inch (about 4.5 mm) of rain, before losses; the rest goes out of the overflow, so route that away from the foundation.
- Is it legal to collect rainwater?
- It depends on where you live. The calculator's own note is that some jurisdictions restrict rainwater capture or require registration, and any connection to household plumbing brings backflow prevention and cross-connection rules. Check the local rules before sizing anything beyond a garden barrel.