Storm Water Runoff Calculator estimates peak flow by Q = CiA, then turns it into runoff volume, detention storage and the smallest storm pipe that carries it in US or metric units.
Enter the intensity for the chosen design storm, for a duration equal to Tc.
Mixed Surfaces (Composite C)
Timing, Volume, Detention & Pipe
Runoff Hydrograph & Storage Diagram
Runoff Coefficient (C) Table
Storm Pipe Capacity Table
Return Period Frequency Factors
Step-by-Step Math
How the Storm Water Runoff Calculator Finds Peak Flow
Stormwater runoff is rain that flows across the ground instead of soaking in. Roofs, pavement and packed soil shed most of the rain that lands on them, while lawns and woods hold much of it back. The Rational Method turns that into a peak flow rate for a small site.
Q is the peak runoff in cubic feet per second (cfs), C the runoff coefficient, i the rainfall intensity in inches per hour and A the drainage area in acres. The units work out because one acre-inch per hour equals 1.008 cfs, close enough to 1 that no conversion factor is used.
For the default single-family lot, C = 0.40, i = 3.5 in/hr and A = 2 acres. That gives Q = 0.40 × 3.5 × 2 = 2.80 cfs, or about 1,257 gallons per minute at the peak of the storm.
In metric, the same formula carries a divisor of 360 when area is in hectares and intensity in mm/hr. One hectare of roofing (C = 0.90) under 50 mm/hr of rain gives Q = 0.90 × 50 × 1 / 360 = 0.125 m3/s, or 125 L/s.
Most rational method tools stop at the peak flow. The Storm Water Runoff Calculator also gives the runoff volume from a whole storm, the detention storage that holds the peak to pre-development levels, and a storm pipe size checked by Manning's equation.
Runoff Coefficient C by Surface
The runoff coefficient is the share of rainfall that becomes runoff at the peak. A C of 0.90 means 90% runs off. The ranges below are the widely used ASCE values, as printed in the Maine DEP stormwater manual and close to the TxDOT hydraulic design manual.
| Surface or Land Use | C Range | Typical C |
|---|---|---|
| Roofs | 0.75 to 0.95 | 0.85 |
| Asphalt and concrete pavement | 0.70 to 0.95 | 0.83 |
| Brick pavement | 0.70 to 0.85 | 0.78 |
| Downtown business | 0.70 to 0.95 | 0.83 |
| Neighborhood business | 0.50 to 0.70 | 0.60 |
| Heavy industrial | 0.60 to 0.90 | 0.75 |
| Light industrial | 0.50 to 0.80 | 0.65 |
| Apartments | 0.50 to 0.70 | 0.60 |
| Multi-unit residential, attached | 0.60 to 0.75 | 0.68 |
| Single-family residential | 0.30 to 0.50 | 0.40 |
| Suburban residential | 0.25 to 0.40 | 0.33 |
| Playgrounds | 0.20 to 0.35 | 0.28 |
| Parks and cemeteries | 0.10 to 0.25 | 0.18 |
| Lawn, heavy soil, 2 to 7% | 0.18 to 0.22 | 0.20 |
| Lawn, sandy soil, 2 to 7% | 0.10 to 0.15 | 0.13 |
| Pasture, heavy soil | 0.15 to 0.45 | 0.30 |
| Pasture, sandy soil | 0.05 to 0.25 | 0.15 |
| Woodland | 0.05 to 0.25 | 0.15 |
Within each range, use the higher value for steep ground, clay soil or a larger storm. The Storm Water Runoff Calculator lets you pick the low, middle or upper value of the range, and the C table inside it lists lawns on sandy and clay soil at three slopes with the source of each row.
Its forest (0.059 to 0.20) and farmland or pasture (0.05 to 0.30) rows follow Lindeburg's Civil Engineering Reference Manual. They sit a little narrower than the Maine woodland and pasture ranges above, so check which table your reviewing agency expects.
Composite C for a mixed site
Real sites mix surfaces, so C is weighted by area. Choose Mixed Surfaces in the Storm Water Runoff Calculator and enter the share of each. A lot that is 25% roof, 15% driveway, 50% clay-soil lawn and 10% woods works out as follows.
That single weighted C then goes into Q = CiA as usual. For a storm sewer serving several areas, add up the C × A of each one and use the total in place of C times A.
Design Storm and Rainfall Intensity
Rainfall intensity is not one number for a town. It depends on how rare a storm you design for and how long the storm lasts. The Rational Method uses the intensity for a storm lasting as long as the time of concentration, Tc, which is how long water takes to travel from the farthest point of the site to the outlet.
In the US, look up intensity or depth in NOAA's Precipitation Frequency Data Server (Atlas 14). Pick your site, the return period and a duration equal to Tc. If NOAA gives a depth, the NOAA Depth option in the Storm Water Runoff Calculator divides it by the duration, so 1.17 in over 20 minutes becomes 3.51 in/hr.
For larger, rarer storms the ground is already wet and less rain soaks in. Design references such as Lindeburg's Civil Engineering Reference Manual raise C by a frequency factor Cf for the peak flow, and the result is capped at 1.00. The calculator applies Cf to the peak only, so runoff volume stays on the base C.
| Design Storm | Frequency Factor Cf | Chance Each Year | Chance in 30 Years | C of 0.40 Becomes | Peak at 3.5 in/hr, 2 ac |
|---|---|---|---|---|---|
| 2-year to 10-year | 1.00 | 50% to 10% | 100% to 96% | 0.40 | 2.80 cfs |
| 25-year | 1.10 | 4% | 71% | 0.44 | 3.08 cfs |
| 50-year | 1.20 | 2% | 45% | 0.48 | 3.36 cfs |
| 100-year | 1.25 | 1% | 26% | 0.50 | 3.50 cfs |
The peak column holds intensity fixed so only the C effect shows. In practice the 100-year intensity is also much higher than the 10-year one, so the real increase in peak flow is larger. A 100-year storm is not once a century, since it has a 26% chance of happening at least once in 30 years.
Time of concentration by Kirpich
If you know the longest flow path and its average slope, the Storm Water Runoff Calculator estimates Tc with the Kirpich equation, with L in feet and S in ft/ft.
An 800 ft flow path at a 2% slope gives Tc = 6.0 minutes. Short Tc values mean intense design rainfall, so getting Tc right matters more than most inputs. The Rational Method is meant for small areas, and many agencies cap it at about 200 acres.
Runoff Volume and Detention Storage
Peak flow sizes pipes and inlets. Volume sizes ponds, cisterns and rain gardens. The Storm Water Runoff Calculator offers two ways to get it. The simple way multiplies the base C, without the frequency factor, by total storm rainfall and area.
On the default 2-acre lot, a 2.5 in storm at C = 0.40 produces 1.00 in of runoff depth. Over 87,120 sq ft that is 7,260 cubic feet, or about 54,300 gallons. The runoff ratio, runoff depth divided by rainfall depth, is 40% here, the same as C.
NRCS curve number method
The second way is the NRCS curve number method, widely used for storm volume. It accounts for the rain the ground absorbs before any runoff starts, so small storms produce little or none. CN runs from about 30 for open woods on sandy soil to 98 for pavement.
With the default CN of 75 and a 2.5 in storm, S = 3.33 in and the initial abstraction Ia = 0.67 in. Runoff depth is (2.5 - 0.67)2 / (2.5 - 0.67 + 3.33) = 0.65 in. Over 2 acres that is about 4,720 cubic feet, or 35,330 gallons.
That is about a third less than the 1.00 in from C times P on the same lot. If the storm is smaller than Ia, 0.67 in here, the method gives no runoff at all, and the tool says so.
For a roof, 1 in of rain on 1,000 sq ft at C = 0.85 sheds 70.8 cubic feet, or about 530 gallons. That is the figure to use for a rain barrel or cistern.
Detention holds back the extra peak that development creates. The tool compares the post-development peak with the peak at a pre-development C, 0.20 by default, and stores the difference using a triangular hydrograph. On the default site that is 2.80 cfs against 1.40 cfs over a 20-minute Tc, about 1,680 cubic feet of storage. Final basin design needs a routed hydrograph, but the figure shows the scale of pond you need.
For the pond itself, the pond calculator gives volume from its shape. If runoff will soak into the ground instead, the soil infiltration rate calculator shows how fast a basin floor drains, and the French drain calculator sizes a gravel trench.
Sizing a Storm Drain Pipe for the Runoff
The Storm Water Runoff Calculator picks the smallest standard pipe that carries the peak flow running full, using Manning's equation for the material and slope you choose. It also reports the velocity and the flattest slope that keeps the pipe moving at 3 ft/s.
| RCP Diameter | Full Capacity at 1% | Full Velocity at 1% | Min Slope for 3 ft/s |
|---|---|---|---|
| 12 in | 3.56 cfs | 4.54 ft/s | 0.44% |
| 15 in | 6.46 cfs | 5.26 ft/s | 0.33% |
| 18 in | 10.5 cfs | 5.94 ft/s | 0.26% |
| 24 in | 22.6 cfs | 7.20 ft/s | 0.17% |
| 30 in | 41.0 cfs | 8.36 ft/s | 0.13% |
| 36 in | 66.7 cfs | 9.44 ft/s | 0.10% |
Values use n = 0.013 for concrete. Smooth PVC (n = 0.011) carries about 18% more, so a 12 in PVC pipe at 1% holds 4.21 cfs. Corrugated metal (n = 0.024) carries a little over half as much, only 1.93 cfs in the same size.
The default 2.80 cfs fits a 12 in RCP at 1%, running 79% full at 4.54 ft/s. Designers usually keep storm sewers between about 3 and 10 ft/s, fast enough to carry grit but slow enough to limit scour. For a partly full pipe, the partial pipe flow calculator gives depth and velocity, and the culvert flow calculator checks a road crossing.
Storm Water Runoff in Practice
Homeowners sizing a rain barrel, contractors laying a yard drain and engineers checking a site plan all start from the same formula. Each answer below works through the numbers for one of those jobs.
How do you calculate stormwater runoff?
Multiply the runoff coefficient by the rainfall intensity and the drainage area. With C in decimal, i in in/hr and A in acres, Q = CiA gives cfs. A half-acre parking lot (C = 0.90) under 4 in/hr of rain sheds 0.90 × 4 × 0.5 = 1.8 cfs, about 808 gallons per minute.
What is the formula for rainfall runoff?
For peak rate, the Rational Method Q = CiA is the standard formula for small sites. For volume, either multiply C by total rainfall depth and area, or use the NRCS curve number equation Q = (P - 0.2S)2 / (P + 0.8S). Both are built into the Storm Water Runoff Calculator, and larger watersheds usually need a full hydrograph model.
What is the formula for runoff depth and volume?
With the simple method, runoff depth equals C times rainfall depth, so 2.5 in of rain at C = 0.40 gives 1.0 in of runoff. With a curve number of 75, the same storm gives 0.65 in. Volume equals runoff depth times area, and one inch of runoff over one acre is 3,630 cubic feet, or about 27,150 gallons.
What is a runoff ratio?
It is the fraction of rainfall that leaves a site as runoff, measured depth over depth. A ratio of 0.40 means 40% of the rain runs off and 60% soaks in, evaporates or is stored. In the Rational Method, the runoff coefficient C plays the same role at the storm peak.
How do you calculate storm drain size?
Find the peak flow with Q = CiA, then size the pipe with Manning's equation for its slope and material. A 6.0 cfs peak at 1% slope needs a 15 in concrete pipe, which carries 6.46 cfs full. Many cities also set a minimum storm sewer size, often 12 or 15 in, regardless of flow.
How do you calculate storm water drainage for a roof?
Use the roof plan area, not the sloped surface, with C between 0.75 and 0.95. A 1,000 sq ft roof under 3 in/hr of rain at C = 0.85 sheds 0.059 cfs, about 26 gpm. Gutter and downspout sizing starts from that flow, and the gutter slope calculator sets the fall along the gutter.
What is storm water runoff?
It is rain or snowmelt that flows over the surface instead of soaking into the ground. It picks up oil, sediment, fertilizer and litter on the way and usually reaches streams through storm drains without treatment. Another name for it is surface runoff or overland flow.
What is the difference between drainage and stormwater?
Stormwater is the water itself, the runoff from rain. Drainage is the system that moves it, such as gutters, inlets, pipes, ditches and ponds. A storm drain carries only rainwater, while a sanitary sewer carries waste from buildings. Most US systems keep the two apart, though some older cities still run combined sewers.
What are the two types of runoff?
Surface runoff flows over the ground and reaches a stream within hours. Subsurface runoff, also called interflow, soaks in and moves sideways through the soil before seeping out, often days later. The Rational Method deals only with surface runoff at the storm peak.
How do you measure runoff on site?
For small areas, catch the flow in a container of known size and time how long it takes to fill. A 5-gallon bucket filling in 10 seconds is 30 gpm. In channels, engineers use a weir or flume with a depth gauge, or measure velocity and cross-section area and multiply the two.
Is there a stormwater calculation spreadsheet formula?
In Excel, put C in A2, intensity in B2 and acres in C2, then enter =A2*B2*C2 for cfs. Multiply by 448.8 for gpm. For runoff volume in cubic feet, use =A2*D2/12*C2*43560 with storm depth in inches in D2.
What is the EPA National Stormwater Calculator?
The EPA National Stormwater Calculator estimates yearly runoff for a US site from local soil and long-term weather records. It answers how much rain a site sheds over a year and how green infrastructure changes that. For a single design storm peak, pipe or detention size, the Rational Method is the usual tool.