Runoff Volume Calculator
One inch of rain on a 1,000 square foot concrete pad does not soak in. It becomes 560.70 gallons of runoff that has to go somewhere. This calculator multiplies rainfall depth, drainage area and a runoff coefficient to give that volume in gallons, so you can size a rain barrel, a drain or a detention basin against a real number instead of a guess.
Volumes are US gallons. A coefficient of exactly 0 is accepted and returns 0.00 gallons: it describes a surface where everything soaks in. The 0.623 factor is the standard rounded conversion (exact value 0.6234 gallons per inch over one square foot).
Quick Answer
One inch of rain over 1,000 square feet of concrete (C = 0.90) produces 560.70 gallons of runoff. The math is 1 × 1,000 × 0.90 × 0.623. Swap in your own storm depth, your own area and the coefficient for your surface from the table below, and the calculator returns the volume your drainage has to handle.
Key takeaways
- Runoff volume = rainfall (in) × area (sq ft) × C × 0.623, answered in US gallons. The coefficient C does most of the work: the same storm makes six times more runoff on concrete than on a lawn.
- One inch over one square foot is 0.623 gallons before losses, so a 1,000 sq ft roof in a 1-inch storm sheds about 623 gallons at C = 1.
- This is a volume tool, not a flow tool. It says how much water arrives in total, not how fast. Pipe and channel sizing needs the peak rate instead.
- For mixed surfaces, run each surface separately and add the volumes. A single blended coefficient hides the roof, which is usually the biggest contributor.
- Quick Answer
- How to use the runoff volume calculator
- The runoff volume formula and the 0.623 factor
- Runoff coefficients by surface: picking C honestly
- Worked example: one inch on 1,000 square feet of concrete
- What competing calculators miss
- Using the number: barrels, drains and basins
- Limits: when this estimate is not enough
- Frequently asked questions
- Add This Calculator to Your Website
How to use the runoff volume calculator
Three inputs, one multiplication. Results stay hidden until you calculate.
- Enter the rainfall depth: the total inches from the storm you are planning for, from a gauge reading or a local design storm. The worked example uses 1 inch.
- Enter the drainage area: length times width in square feet. For land measured in acres, one acre is 43,560 sq ft; an acreage calculator helps if the parcel is irregular.
- Enter the runoff coefficient: pick C from the surface table below. When a range is given, use the high end for steep, saturated or clay ground and the low end for flat, dry, sandy ground.
- Click Calculate runoff volume: the result line reads, for example, “Runoff Volume: 560.70 gallons”. Copy or share it from the results box.
Every invalid entry gets the same rejection: “Please enter valid positive values for all fields.” That covers blanks, text, zero or negative rainfall, zero or negative area, and any coefficient below 0 or above 1. One exception is worth knowing: a coefficient of exactly 0 is valid and returns 0.00 gallons, because a fully absorbent surface genuinely produces no runoff in this model.
The runoff volume formula and the 0.623 factor
The formula is a volume version of the Rational Method idea: rainfall depth spread over an area, scaled by the share that runs off. Depth times area is a volume in inch-square-feet; the 0.623 factor converts that to gallons.
Where the exact conversion is 7.48052 gallons per cubic foot ÷ 12 inches per foot = 0.6234 gallons per inch over one square foot, rounded to 0.623.
The rounding costs about 0.06% against the exact factor, far less than the uncertainty in C itself, and 0.623 is the constant the rainwater-harvesting and stormwater references use, according to the USGS Water Science School runoff overview and standard extension guidance. What the formula leaves out matters as much: it uses total storm depth, not intensity, and it folds infiltration, evaporation and initial wetting losses into C rather than modeling them separately. The US Environmental Protection Agency makes the same simplification in screening-level tools before moving to simulation models such as the EPA Storm Water Management Model (SWMM) for design work.
Runoff coefficients by surface: picking C honestly
C is the share of rainfall that becomes runoff, and it is the input people most often get wrong. A roof and a forest in the same storm differ by a factor of nine, so a guessed C swamps every other number on the page.
| Surface | C range | Reading the range |
|---|---|---|
| Roofs | 0.75 to 0.95 | Metal and shingle roofs sit high; use the top end for steep pitches that shed fast. |
| Concrete and asphalt | 0.80 to 0.95 | Effectively waterproof. Cracked, aged pavement with gravel joints sits at the low end. |
| Gravel drives and paths | 0.40 to 0.60 | Depends on the base: loose gravel over sand absorbs far more than compacted gravel over clay. |
| Lawns, flat with good soil | 0.10 to 0.30 | Dry sandy lawns at the low end; wet clay lawns behave almost like pavement in a long storm. |
| Steep grass slopes | 0.30 to 0.50 | Water moves downhill before it can soak in, so slope alone can double a lawn’s coefficient. |
| Forest and woodland | 0.10 to 0.20 | Canopy, litter and root channels absorb most storms; only long or intense rain produces much runoff. |
Slope, soil and saturation move a surface inside its range. A lawn that is already saturated from yesterday’s storm has nowhere to put today’s water, which is why drainage designers bias C upward for design storms. If you need the soil side of that judgment, the rate at which water enters the ground is its own measurement: see the soil infiltration rate calculator for the test-based version.
Worked example: one inch on 1,000 square feet of concrete
The pre-filled values are a 1-inch storm over a 1,000 sq ft concrete pad or roof at C = 0.90. Running the calculator gives 560.70 gallons, about ten standard 55-gallon rain barrels from a single modest storm.
Hold the storm fixed at 1 inch over 1,000 sq ft and only the surface changes. The spread below is the whole argument for taking C seriously:
Four more runs of the same tool, checked against the formula, cover the cases readers actually size things with:
| Case | Rainfall | Area | C | Runoff volume |
|---|---|---|---|---|
| Grass field, moderate storm | 2 in | 2,500 sq ft | 0.30 | 934.50 gal |
| House roof, 1-inch storm | 1 in | 1,500 sq ft | 0.90 | 841.05 gal |
| Garden, light rain | 0.5 in | 2,000 sq ft | 0.25 | 155.75 gal |
| One acre of paving, 3-inch storm | 3 in | 43,560 sq ft | 0.95 | 77,342.96 gal |
What competing calculators miss
Runoff tools cluster into a few types, and each leaves a gap this page closes:
- Rate tools wearing a volume label (Calculator Ultra): its runoff page presents a garbled formula that divides by 12 and then by 7.481 with no runoff coefficient at all, so a roof and a meadow of the same size return the same answer. Dropping C drops the physics.
- Roof-harvesting tools (roof pitch and rain-collection blogs): the math is right for roofs (0.623 included), but the framing is collection only: no coefficient table for ground surfaces, no detention or drainage use, and storage advice that assumes every gallon can be captured.
- Landscaper stormwater tools (WeldPac): same 0.623 formula plus an impact label, but the coefficient guidance stops at “higher for driveways, lower for grass” with no ranges to pick from, and soil absorption is an optional extra input the formula never clearly uses.
- Engineering Rational Method tools (Q = C I A calculators): correct and thorough, but they answer a different question, peak flow in cubic feet per second for pipe sizing, and bury the homeowner who just wants gallons for a barrel or a basin.
- The gap this page fills: the volume formula with a sourced coefficient table, the mixed-surface method, an honest note on what total depth ignores (intensity), and runs you can reproduce in the tool itself, including the corrected 1,500 sq ft roof figure of 841.05 gallons.
Using the number: barrels, drains and basins
A volume estimate earns its keep in three jobs. Each one uses the same gallons figure differently.
Rain barrels and cisterns. Divide the storm volume by the vessel size. The worked example’s 560.70 gallons overflows a single 55-gallon barrel ten times over, which is why serious harvesting on a 1,000 sq ft roof starts at several hundred gallons of storage, or accepts that barrels capture only the first part of a storm. The EPA’s Soak Up the Rain program makes the same point at property scale: capture at the source, sized to the roof, beats downstream fixes.
Drainage and conveyance. Volume tells you how much storage a French drain trench, dry well or detention basin must hold; it does not size the pipe that fills it, because pipes care about flow rate. Pair this page with the drain flow rate calculator for the rate side, and budget a French drain cost estimate once the trench volume is known. Gutter capacity is its own check: a roof plane draining to one downspout concentrates the whole volume at one point.
Mixed surfaces. Real sites are patchworks. Run the roof, the drive and the lawn as three separate calculations and add the gallons. If you need a single blended coefficient for a report, weight by area: C = (C1 × A1 + C2 × A2 + …) ÷ total area. A 1,500 sq ft roof at 0.90 plus a 3,500 sq ft lawn at 0.25 blends to (1,350 + 875) ÷ 5,000 = 0.445, but the split runs (841.05 gal + 545.13 gal at 1 inch = 1,386.18 gal) show the roof contributing 61% of the runoff from 30% of the area.
Limits: when this estimate is not enough
The formula is a screening estimate, and three limits decide whether it is the right tool. First, it ignores rainfall intensity: two inches in twenty minutes and two inches over a day give the same volume here, but very different flooding. Design storms come from NOAA precipitation frequency data, and anything protecting a building deserves that check. Second, it treats C as constant through the storm, while real surfaces absorb early rain and shed later rain, so long storms on pervious ground run higher than the table suggests. Third, scale: for catchments beyond small sites, the standard methods are the USDA Natural Resources Conservation Service procedures in Technical Release 55 and the FHWA urban drainage manual (HEC-22), which model hydrographs rather than a single multiplication. Use this page to size barrels, sanity-check a contractor’s basin, and understand your site. Use the engineering methods to stamp a design.
Frequently asked questions
Multiply the rainfall depth in inches by the drainage area in square feet, then by the runoff coefficient, then by 0.623. One inch over 1,000 square feet of concrete at C = 0.90 is 1 × 1,000 × 0.90 × 0.623 = 560.70 gallons. The calculator runs the same multiplication and rejects coefficients outside 0 to 1.
Roofs run 0.75 to 0.95, concrete and asphalt 0.80 to 0.95, gravel 0.40 to 0.60, flat lawns 0.10 to 0.30, steep grass 0.30 to 0.50 and forest 0.10 to 0.20. Inside a range, pick the high end for steep, wet or clay ground and the low end for flat, dry, sandy ground.
It converts inch-square-feet into gallons. One inch of water over one square foot is 1/12 of a cubic foot, and one cubic foot holds 7.48052 gallons, so each inch over a square foot is 7.48052 ÷ 12 = 0.6234 gallons, rounded to 0.623. The rounding understates volume by about 0.06%.
It means none of the rain runs off: the surface absorbs everything, and the calculator returns 0.00 gallons. Zero is accepted as a valid coefficient even though the error message asks for positive values; rainfall and area, by contrast, must be greater than zero.
Run each surface separately with its own area and coefficient, then add the gallons. A 1,500 sq ft roof at C = 0.90 gives 841.05 gallons in a 1-inch storm and a 3,500 sq ft lawn at C = 0.25 gives 545.13 gallons, for a site total of 1,386.18 gallons. A single blended coefficient hides how much the roof dominates.
Not directly: it takes inches and square feet only. Convert first: divide millimetres by 25.4 to get inches, and multiply square metres by 10.764 to get square feet. In metric the shortcut is simpler: 1 mm of rain over 1 square metre is exactly 1 litre before the coefficient is applied.
No. Volume is the total gallons from the whole storm; peak rate is how fast water arrives at the worst moment, usually from Q = C × I × A with rainfall intensity I. Volume sizes storage such as barrels and basins. Rate sizes pipes, channels and culverts.
Treat it as a planning estimate. The coefficient is a range, not a constant, so the same site can honestly vary by 20% or more depending on slope, soil and how wet the ground already is. For a basin that protects a building, confirm the design storm in NOAA precipitation frequency data and have the final sizing checked against NRCS or FHWA drainage methods.
Add This Calculator to Your Website
Embed this calculator on your own site with the code below. Pick a size to preview it live, then copy the code.
Powered by Calculator Zilo
References
- U.S. Geological Survey, Water Science School. Runoff: Surface and Overland Water Runoff (what runoff is, and the land use, soil, slope and saturation factors that drive it). Accessed October 24, 2026.
- U.S. Department of Agriculture, Natural Resources Conservation Service. Technical Release 55: Urban Hydrology for Small Watersheds (1986), curve-number tables as reproduced in the U.S. Army Corps of Engineers HEC-HMS documentation (the NRCS curve-number runoff method this simple volume estimate defers to at design scale). Accessed October 24, 2026.
- Federal Highway Administration. Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22, Third Edition (Rational Method application and runoff coefficient selection for drainage design). Accessed October 24, 2026.
- U.S. Environmental Protection Agency. Soak Up the Rain (property-scale stormwater capture, rain barrels and rain gardens sized against roof runoff). Accessed October 24, 2026.
- U.S. Environmental Protection Agency. Storm Water Management Model (SWMM) (the simulation model used when a screening volume estimate is not enough). Accessed October 24, 2026.
- National Oceanic and Atmospheric Administration, National Weather Service. Precipitation Frequency Data Server (NOAA Atlas 14) (design-storm rainfall depths by location and return period). Accessed October 24, 2026.