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View all 5 tools →Rainwater Harvesting Calculator
Size a cistern against your roof's annual collection potential and your garden's real weekly water demand, so you know days of supply, not just a gallon total.
System Sizing
1. Roof catchment and your region's annual rainfall
Use your region's long-term average, not one year -- NOAA's climate normals or a local extension office publish this for most US locations.
2. Your cistern or storage tank
This is a cistern or tank sized in hundreds to thousands of gallons, not a 50 gal rain barrel -- for a single barrel and single-storm overflow, see the Rain Barrel Calculator instead.
3. What you're watering with it
1–1.5 in a week is the typical outdoor watering range extension programs cite for lawn and garden beds; adjust for your own plants and climate.
4. Indoor non-potable use (optional)
20–25 gal/person/day is a commonly cited rough combined estimate for toilet flushing and laundry through a non-potable line -- plumbing codes require this to be a fully separate system from your potable water, not merely optional.
A rain barrel answers "will tonight's storm overflow the barrel I own." This calculator answers a bigger question: over a typical year in your area, how much of your outdoor watering could a properly sized cistern actually replace. Enter your roof, your region's average annual rainfall and your storage tank size, and it compares annual collection potential against a garden's real weekly water demand, so a cistern size means days of supply, not just a large number of gallons.
Reviewed by Usama, Civil EngineerHow we source our numbers
How it works
- Enter your roof's catchment area and your region's long-term average annual rainfall, the figure NOAA climate normals or your local extension office publish, not a single year's total.
- Enter your cistern or storage tank size in gallons, sized in the hundreds or thousands rather than one 50 gallon barrel.
- Enter the garden or bed area you're watering and a weekly watering depth, and the calculator works out weekly demand, how many days a full cistern supplies that demand, and what share of a full year's demand your roof's annual collection could realistically cover.
Typical runoff coefficient by roof surface
| Roof surface | Runoff coefficient |
|---|---|
| Metal (standing seam or sheet) | 90% |
| Asphalt shingle | 80% |
| Tile (clay or concrete) | 83% |
| Flat / low-slope membrane (EPDM, TPO) | 80% |
The same catchment formula, applied at a different scale
Annual gallons collected equals roof area in square feet, multiplied by your region's average annual rainfall in inches, multiplied by 0.623, multiplied by a runoff coefficient of roughly 0.8 to 0.9 depending on roof material. This is the identical formula the site's Rain Barrel Calculator uses for a single storm, published in the Texas Water Development Board's "Texas Manual on Rainwater Harvesting" -- the only thing that changes at this scale is the rainfall figure, an annual regional average instead of one storm's total, and what you do with the result: sizing a cistern against ongoing demand rather than checking whether one barrel will overflow tonight.
Why a demand-side comparison matters more than a raw gallon total
A roof that collects 20,000 gallons a year sounds impressive until you know whether that is 40% or 400% of what a garden actually needs. This calculator converts a garden's weekly watering depth into a gallons-per-week demand figure using the same 0.623 gal/sqft-inch constant as the collection side, then divides the cistern size by that weekly demand to show days of supply -- a number you can actually plan an irrigation schedule around, rather than an abstract annual total that says nothing about whether the tank will still have water in it during a dry August.
Why rainfall does not arrive evenly, and what that means for sizing
A cistern that covers 100% of a garden's annual demand on paper can still run dry mid-summer, because rain does not fall in equal weekly installments -- most regions get a disproportionate share of their annual total in a handful of wet months, then go weeks with little or none during peak growing season, which is exactly when demand peaks too. The percent-of-annual-demand figure this calculator shows is a useful sizing anchor, not a guarantee of year-round supply; a cistern sized at 100% of annual demand on paper is a reasonable starting point precisely because it is expected to run short during the driest stretch, not because it is oversized.
How this differs from a rain barrel
A rain barrel is a small, opportunistic catch sized around a single storm, typically 50 gallons, and it is expected to overflow in anything beyond light rain -- that is the question the site's Rain Barrel Calculator answers. A rainwater harvesting system is a cistern sized against a full season of rainfall and a full season of demand, meant to actually offset a meaningful share of outdoor water use rather than catch a first flush. If your storage is a single retail rain barrel and your question is whether tonight's storm overflows it, use the Rain Barrel Calculator instead; use this tool once you are sizing real storage against real demand.
Further reading
Frequently asked questions
How do I calculate rainwater harvesting potential?
Multiply your roof's catchment area in square feet by your region's average annual rainfall in inches, by 0.623, by a runoff coefficient of roughly 0.8 to 0.9 depending on roof material. This is the formula published in the Texas Water Development Board's Texas Manual on Rainwater Harvesting. Enter your own roof area and regional annual rainfall above for an exact annual figure.
How big should my rainwater harvesting tank be?
Size it against your actual weekly water demand, not just your roof's collection potential. This calculator divides your cistern size by your garden's weekly demand to show days of supply from a full tank, which is a far more useful sizing number than annual gallons collected alone. A tank sized around 2 to 4 weeks of supply is a common practical target for a garden without full irrigation automation, though the right number depends on how often your region actually gets rain to refill it.
How much rainwater can I collect in a year?
Roughly your roof area in square feet, times your region's average annual rainfall in inches, times 0.623, times a runoff coefficient around 0.8 to 0.9. A 1,800 square foot roof in a 35 inch annual rainfall region at a 0.8 coefficient collects about 31,395 gallons a year. Enter your own figures above for a number specific to your roof and region.
How do I find my region's average annual rainfall?
NOAA's climate normals are the standard US reference, and most state and county extension offices publish the same figures in a more local format. Use a long-term average rather than last year's total, since a single year can run well above or below the typical figure and would size a cistern wrong in either direction.
How much water does a garden need per week?
University of Illinois Extension and Clemson Cooperative Extension both put typical outdoor lawn and garden bed watering in a 1 to 1.5 inch per week range to maintain healthy growth. This calculator uses that range as a default, fully editable for your specific plants, soil and climate.
Will a rainwater harvesting system cover all my garden's water needs?
It depends on your roof size, your region's rainfall and your garden's demand, and even a system sized to cover 100% of annual demand on paper can still run short mid-summer, since rainfall does not arrive in even weekly installments. This calculator shows the percent of annual demand your setup could realistically cover, which is a planning anchor, not a guarantee of never running dry.
What size cistern do I need for irrigation?
Work backward from days of supply you want between refills, then multiply by your garden's weekly demand. This calculator does that math directly: enter your garden area and watering depth, and it shows how many days any given cistern size supplies, so you can size up or down until the days-of-supply figure matches how often you're comfortable relying on rain to refill it.
How is rainwater harvesting different from a rain barrel?
Scale and purpose. A rain barrel is a small, roughly 50 gallon catch sized around a single storm, and it is expected to overflow in anything beyond light rain -- that's what the site's Rain Barrel Calculator is for. A rainwater harvesting system is a cistern sized against a full season of rainfall and real irrigation demand, meant to offset a meaningful share of outdoor water use over weeks or months, not just catch a first flush.
Does roof material affect rainwater harvesting potential?
Yes. A smooth metal roof runs close to a 0.9 runoff coefficient, while a more porous asphalt shingle roof runs closer to 0.8, a roughly 10% difference in annual collection for the same roof area and rainfall. This calculator's roof type picker applies that difference directly rather than assuming one fixed efficiency.
Is harvested rainwater safe to drink?
This calculator does not address water quality or treatment; it sizes collection and storage against irrigation demand only. Harvested rainwater used for drinking typically requires filtration and disinfection well beyond what a simple cistern provides, and local plumbing codes usually require harvested water to be clearly labeled as non-potable and kept separate from a home's regular water supply.
What is a good size roof for rainwater harvesting?
There is no fixed minimum; the calculation scales linearly with roof area, so a smaller roof simply collects proportionally less. What matters more for whether a system is worth building is the ratio between your roof's annual collection potential and your actual garden demand, which is exactly the percent-of-annual-demand figure this calculator shows -- a small roof over a small garden can cover a larger share of demand than a big roof over an even bigger one.