Rainfall Catchment & Payback Reference
Six common roof sizes run through the Rain Barrel Calculator, Irrigation Timing Calculator, and Water Savings Calculator math, so you can see the shape of the numbers before entering your own roof.
Roof size, harvest, storage, and an example payback
| Roof profile | Roof area | Gallons per inch of rain | Liters per inch | 55-gal barrels needed | Overflow past 1 barrel | Annual harvest* | Days of garden watering per inch† | Example annual saving‡ |
|---|---|---|---|---|---|---|---|---|
| Garden shed or small porch roof | 200 sq ft | 105.91 gal | 400.91 L | 2 | 50.91 gal | 3,177 gal | 4 | $20.65 |
| Single-car garage roof | 400 sq ft | 211.82 gal | 801.83 L | 4 | 156.82 gal | 6,355 gal | 7.9 | $41.30 |
| Small single-story home | 1,000 sq ft | 529.55 gal | 2,004.56 L | 10 | 474.55 gal | 15,887 gal | 19.8 | $62.48 |
| Average single-story home | 1,500 sq ft | 794.33 gal | 3,006.87 L | 15 | 739.33 gal | 23,830 gal | 29.8 | $62.48 |
| Two-story home | 2,200 sq ft | 1,165.01 gal | 4,410.04 L | 22 | 1,110.01 gal | 34,950 gal | 43.6 | $62.48 |
| Large home, multiple roof sections | 3,200 sq ft | 1,694.56 gal | 6,414.61 L | 31 | 1,639.56 gal | 50,837 gal | 63.5 | $62.48 |
* Annual harvest assumes an example 30 inches of rain a year at a 0.85runoff coefficient — the same default the Rain Barrel Calculator uses. Replace it with your own region’s annual rainfall for a real figure.
† Measured against one fixed reference garden (300 sq ft, a 1-inch-a-week target, 2 sessions), computed by the Irrigation Timing Calculator: about 187 gallons a week.
‡ Illustrative only, at an example $6.50 per 1,000 gallons — not a real utility rate. See the FAQ below for why several rows land on the same figure.
How to read this table
Every column here traces back to the same three constants and formulas the calculators use. Gallons per inch of rain comes from the roof’s footprint area multiplied by 0.623 gallons per square foot per inch (the fixed conversion behind the Rain Barrel Calculator) and the 0.85 runoff coefficient it defaults to. Barrels needed is that harvest divided by a 55-gallon barrel, rounded up. Days of garden watering divides that same per-inch harvest by the reference garden’s daily demand, from the Irrigation Timing Calculator’s own math.
The annual-harvest and annual-saving columns work the same way, just run at an example 30-inch annual rainfall total instead of a single inch, and passed through the Water Savings Calculator with the reference garden’s monthly demand as the baseline being offset. That’s why the smallest two roofs (shed, garage) show a partial saving that scales with their harvest, while the larger roofs plateau at the same number: their annual harvest already exceeds what the fixed reference garden could ever use, so the calculator has nothing further to offset in this specific scenario. A real household with a bigger garden, an outdoor pool top-up, or a second demand to offset would see that plateau shift.
Overflow matters as much as harvest here. Even the smallest roof profile clears a single 55-gallon barrel in well under a full inch of rain, and every profile above it overflows a single barrel many times over. Treat the barrels-needed column as a planning number for total storage, not a suggestion to actually chain that many barrels together — an overflow hose routed away from the foundation, and a realistic sense of how much storage you’ll actually use, usually matters more than trying to catch every last gallon.
Frequently Asked Questions
Where do these numbers come from?
Every gallons, barrels, days-of-irrigation, and dollars figure in the table is computed by calling the exact same calcRainBarrelHarvest(), calcIrrigationTiming(), and calcWaterSavings() functions that power the Rain Barrel, Irrigation Timing, and Water Savings calculators on this site — nothing here is a separately hand-typed table. Change your roof's real numbers in the calculators themselves for a figure specific to your house.
Why is the annual saving the same for several of the larger roofs?
The saving is calculated against one fixed reference garden's annual demand. Once a roof's yearly harvest is large enough to fully cover that garden's needs, a bigger roof still harvests more water — you can see that in the annual-harvest and barrels-needed columns — it just doesn't create a bigger dollar saving against this specific, fixed demand. In practice a bigger roof mostly means more overflow to plan for, not more savings, unless it's paired with a bigger garden or more storage uses.
Is 30 inches a real annual rainfall figure for my area?
No — it's a round, clearly-labeled example so the annual-harvest and annual-saving columns have a concrete number to show. Actual annual rainfall varies enormously by region, from arid climates well under 15 inches a year to temperate or coastal regions well over 50. Swap in your own region's known annual total and rerun the Rain Barrel Calculator directly for a figure that means something for where you live.
Is $6.50 per 1,000 gallons a real water rate?
No — it's an illustrative example, not a sourced utility rate. Water (and any sewer surcharge billed alongside it) rates vary widely by utility and region and change over time. Use the Water Savings Calculator with the rate from your own bill for a figure that applies to you.
Planning reference only. Rainwater collection rules, and how much storage is practical or permitted, vary by jurisdiction — check locally before installing a system. Harvested rainwater from a roof is for outdoor and non-potable use.
Use it with
- Rain Barrel Calculator — run your own roof area, local rainfall, and barrel count instead of one of the six reference profiles above.
- Irrigation Timing Calculator— replace the reference garden’s 300 sq ft and 1-inch-a-week target with your own bed or lawn’s numbers.
- How Big a Rain Barrel (or Cistern) Do You Actually Need? — the guide version of this table, walking through how to match storage to your own roof and garden.