Free rainfall-to-gallons calculator
Enter the rainfall depth and your catchment area to see how much water a roof or surface collects — in gallons, liters, and cubic feet — the first number any rainwater harvesting setup needs, updated live, as you type.
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Estimates assume 100% collection efficiency. Real-world yield is typically 75–90% of the theoretical maximum due to first-flush losses, evaporation, and surface absorption. Check local regulations before installing a rainwater harvesting system.
Results are estimates. Consult a professional.
What the rainfall-to-gallons calculator does
The rainfall-to-gallons calculator turns a depth of rain and a catchment area into the volume of water that falls on it. Give it how many inches fell and how many square feet the surface covers — a roof, a patio, a garden bed, a parking lot — and it returns the harvest in US gallons, with liters and cubic feet alongside. It is the first number anyone planning a rainwater harvesting setup needs: how much water a given roof can actually collect from a storm.
The volume comes from a simple physical fact: rain falls evenly across a flat surface, so the water it deposits is just the depth of the rain multiplied by the footprint it landed on. A bigger roof or a heavier downpour means more gallons, in direct proportion. The calculator does that multiplication and the unit conversion for you, so you can size barrels, cisterns, and downspouts against a real figure instead of a guess.
How rainfall converts to gallons
Picture the rain as a thin sheet of water sitting on the catchment surface. Its volume is the area of the surface times the thickness of the sheet — the rainfall depth. Multiply those two and you have the volume in cubic feet; convert cubic feet to gallons and you have the harvest. The calculator does both steps at once.
The inputs explained
Only two numbers go in, and both are easy to read off a weather report or a tape measure. Understanding what each one does to the result makes the harvest easy to estimate in your head.
Rainfall depth
How many inches of rain fell — the figure a rain gauge or weather report gives. It scales the result directly: double the depth and you double the gallons. Use the total for a single storm to size a downpour, or an average monthly or annual total to plan storage across a season.
Catchment area
The flat footprint the rain lands on, in square feet. For a roof this is the horizontal area it covers — its footprint as seen from above — not the sloped surface area, because a pitched roof catches the same rain as the flat ground beneath it. Measure the building's footprint, or the length times the width of any flat catchment, and enter that.
Rainwater harvest by roof size and rainfall
This table gives the theoretical harvest in US gallons for common roof footprints and storm sizes. Read down to your catchment area, across to the rainfall depth. Each figure is the full volume that lands on the surface — see the next section for how much of it you actually capture.
| Catchment area | 0.5 in rain | 1 in rain | 2 in rain |
|---|---|---|---|
| 500 ft² | 156 | 312 | 623 |
| 1,000 ft² | 312 | 623 | 1,247 |
| 1,500 ft² | 468 | 935 | 1,870 |
| 2,000 ft² | 623 | 1,247 | 2,494 |
Theoretical harvest in US gallons, from gallons = depth (in) × area (ft²) × 0.6233. These are gross volumes before collection losses; plan usable storage against roughly 75–90% of the figure shown.
Collection efficiency and first-flush losses
The calculator returns the full volume of rain that lands on the catchment. You never collect quite all of it. Some evaporates off a warm roof before it runs off, some clings to the surface and never reaches the gutter, some splashes over the edges in a hard downpour, and the gutters and pipes leak a little. In practice a well-built system captures about 75–90% of the theoretical figure.
On top of the steady losses, most systems deliberately throw away the first flush — the dirtiest water of a storm, which rinses dust, pollen, leaf litter, and droppings off the roof before clean water arrives. A first-flush diverter holds that initial runoff back so it never enters your tank. Texas A&M AgriLife suggests sizing the diverter for roughly 1–2 gallons per 100 ft² of roof, so a 1,000 ft² roof discards its first 10–20 gallons each storm.
The first-flush sizing rule — about 1–2 gallons of diversion per 100 ft² of catchment area — is Texas A&M AgriLife Extension's recommendation for pre-storage treatment.A worked example: a 1-inch storm on a 1,000 ft² roof
Dana wants to capture roof runoff for the garden. The house has a 1,000 ft² footprint, and a typical local storm drops about 1 inch of rain. How much water is that, and how many barrels would it fill?
Step 1 — Enter the depth and area
Rainfall depth 1 inch, catchment area 1,000 ft². Both are the values the calculator opens with, so this is the default case.
Step 2 — Multiply through the formula
Using gallons = depth × area × 0.6233 gives 1 × 1,000 × 0.6233 = 623 gallons. The calculator also shows this as 2,360 liters and 83.3 cubic feet.
Step 3 — Apply real-world efficiency
At roughly 80% collection efficiency, about 500 gallons actually reaches storage after roof losses and the first-flush diversion. That single storm would overflow a standard 55-gallon rain barrel many times over.
Practical tips for sizing a harvesting system
- Measure the footprint, not the slope. A pitched roof collects the rain that falls on its horizontal shadow, so use the building's footprint area. Adding the sloped surface area overstates the harvest.
- Storage is the real constraint, not the roof. Even a small roof overruns a single barrel in one storm. Size your cistern to the rain you want to keep between storms, then link barrels or step up to a tank.
- Plan against ~80% of the calculated number. Roof losses, splash, leaks, and the first-flush diversion all skim off the top. Designing to the gross figure leaves no margin.
- Use annual rainfall for sizing, single storms for overflow. Average yearly rainfall tells you total supply; the biggest expected storm tells you how fast water arrives and whether your gutters and overflow can handle it.
- Treat before drinking. Roof runoff carries dust, droppings, and debris. Harvested rainwater is fine for gardens and flushing as-is, but potable use needs filtration plus disinfection.
Working in the other direction — from the weight of accumulated precipitation rather than its volume — see the snow load calculator, which estimates the load that snow places on the same roof footprint.
How accurate is the rainfall-to-gallons calculator?
The arithmetic itself is exact. Volume is depth times area, and the unit conversions — 1 inch is 1/12 of a foot, 1 cubic foot is 7.480519 US gallons — are fixed constants, so the gallons, liters, and cubic feet the calculator returns are precise to the inputs you give it.
The uncertainty lives in the real world, not the math. Your harvest depends on how accurately you measured the catchment area, how much rain actually fell versus the forecast, and the collection efficiency of your particular roof and gutters. Treat the calculator's output as the theoretical maximum — the full volume of water that lands on the surface — and apply a 75–90% efficiency factor, plus the first-flush diversion, to estimate what you will keep. For garden and storage planning that is all the precision the job needs.
Frequently asked questions about the free rainfall-to-gallons calculator
About this rainfall-to-gallons calculator
This rainfall-to-gallons calculator runs entirely in your browser — nothing you enter is sent anywhere. It multiplies rainfall depth by catchment area and converts the result to US gallons, liters, and cubic feet (using 1 cubic foot = 7.480519 gallons), giving the theoretical volume of water a roof or surface collects from a storm.
It pairs with the snow load calculator for the weight of precipitation on the same roof. Browse every weather tool on the calculators index.