Estimate peak storm water runoff from a site using the Rational Method, Q = CiA. Break the site into up to several sub-areas (roof, pavement, lawn, etc.), each with its own runoff coefficient, and the calculator works out a composite (area-weighted) runoff coefficient, then combines it with a design rainfall intensity you supply to get peak flow. A Kirpich time-of-concentration estimate is included to help you pick an appropriate intensity from your local IDF (intensity-duration-frequency) curve.
Switch between metric (hectares, mm/hr, m³/s) and imperial (acres, in/hr, cfs) units at the top of the form; every result converts with it.
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The Rational Method is only accepted for small drainage areas (most manuals cap it around 200 acres / 80 ha). Not a substitute for a full hydrology/hydraulics study on larger or complex sites.
How the calculation works
- Composite runoff coefficient. Ccomposite = Σ(Ci·Ai) / ΣAi, area-weighted across every sub-area you enter.
- Time of concentration. Estimated with the Kirpich formula from the longest flow-path length and its average slope, giving a starting point for choosing a design storm duration.
- Peak flow. Q = C·i·A (imperial, i in in/hr, A in acres, Q in cfs) or Q = C·i·A/360 (metric, i in mm/hr, A in hectares, Q in m³/s), using the rainfall intensity you enter for the chosen storm duration and return period.
What to have ready
- Area and runoff coefficient for each land-cover type on the site (roof, pavement, landscaping, etc.)
- Longest overland/channel flow path length and its average slope
- A design rainfall intensity for your location, storm duration and return period, from a local IDF curve or design standard
Notes and limitations
The Rational Method is intended for small drainage areas (typically under about 200 acres / 80 hectares) with reasonably uniform runoff characteristics; larger or more complex watersheds need a hydrograph-based method. There is no single national rainfall-intensity formula — you must supply an intensity value from your local jurisdiction’s IDF curves, NOAA Atlas 14, or equivalent source; this tool does not generate one for you. Runoff coefficients are typical published values and should be adjusted for local soil, slope and land-use conditions. Final acceptance of the design rests with the responsible engineer.
Rational Method Runoff Coefficient (C) Reference Table
| Surface Type / Land Use | Runoff Coefficient C (min) | Runoff Coefficient C (max) | Notes |
|---|---|---|---|
| Pavement (asphalt / concrete) | 0.70 | 0.95 | Nearly impervious; use 0.95 for design |
| Rooftop (flat or sloped) | 0.75 | 0.95 | Depends on membrane and ponding |
| Gravel (packed) | 0.25 | 0.60 | Depends on base and compaction |
| Lawns — sandy soil, flat (<2%) | 0.05 | 0.10 | High infiltration capacity |
| Lawns — heavy clay, steep (>7%) | 0.25 | 0.35 | Low infiltration; high runoff |
| Cultivated fields — row crops | 0.25 | 0.40 | Soil type and row orientation matter |
| Forest / Wooded areas | 0.05 | 0.25 | Litter layer absorbs rainfall |
| Commercial district (downtown) | 0.70 | 0.95 | High impervious area |
| Residential — single family (1/4 ac lots) | 0.25 | 0.40 | Mixed lawns and impervious surfaces |
| Industrial (light) | 0.50 | 0.80 | Mix of rooftops, pavement, and landscaping |
Source: ASCE/EWRI 36-15 (Rational Method), ASCE Manual of Engineering Practice No. 36, and FHWA HDS-2. Values are typical ranges — confirm with local stormwater design manual.
Storm Water Runoff Calculator (Rational Method) FAQ
What is the Rational Method and when is it applicable?
The Rational Method is a widely used hydrological formula — Q = C × i × A — that calculates peak stormwater runoff flow rate (Q) in cubic feet per second (cfs) from the runoff coefficient (C), rainfall intensity (i) in inches per hour, and drainage area (A) in acres. It was developed in the mid-19th century and is codified in ASCE Manual of Engineering Practice No. 36 and FHWA HDS-2. The Rational Method is most accurate for small, urban watersheds under 200 acres with relatively uniform land use and time-of-concentration under 60 minutes. For larger watersheds or complex topography, the Modified Rational Method, TR-55, or HEC-HMS are more appropriate.
What is the time of concentration (Tc) and how do I calculate it?
The time of concentration (Tc) is the time it takes for runoff to travel from the most hydraulically remote point in a watershed to the outlet. It governs which rainfall intensity is used in the Rational Method: a longer Tc uses a lower average rainfall intensity from the IDF curve, and a shorter Tc uses a higher intensity. Tc is the sum of overland flow time, shallow concentrated flow time, and channel flow time, each calculated using equations from TR-55 Chapter 3 or ASCE HDS-2. For urban sites, Tc is often 5–30 minutes; minimum design Tc is typically 5 minutes per local stormwater manuals.
What is a rainfall Intensity-Duration-Frequency (IDF) curve?
An IDF curve is a plot of rainfall intensity (in/hr) vs. storm duration for various return periods (2-yr, 10-yr, 25-yr, 100-yr). The intensity used in the Rational Method is read from the IDF curve at the time of concentration (Tc) and the selected design storm return period. IDF data is available from the NOAA Atlas 14 (hdsc.nws.noaa.gov/ha2tp2/) for all U.S. locations. For example, in Atlanta, Georgia, the 10-year, 60-minute rainfall intensity is approximately 3.6 in/hr; in Phoenix, Arizona, it is approximately 2.0 in/hr. Local stormwater design manuals provide region-specific IDF curves that take precedence.
How do I combine runoff coefficients for a mixed-use watershed?
When a drainage area has multiple surface types (e.g., parking lot, lawn, and building rooftop), use a weighted composite runoff coefficient: C_composite = Σ(Ci × Ai) / A_total, where Ci is the runoff coefficient for each sub-area and Ai is its area. For example, a 5-acre site with 2 acres of pavement (C = 0.90), 2 acres of lawn (C = 0.25), and 1 acre of rooftop (C = 0.90) has: C_composite = (2×0.90 + 2×0.25 + 1×0.90) / 5 = (1.80 + 0.50 + 0.90) / 5 = 0.64. This composite C is then used with the site-wide rainfall intensity and total area in Q = C × i × A.
What return period should I use for stormwater design?
The design return period (also called recurrence interval or frequency) depends on the consequences of overflow and local code requirements. Common design standards: minor storm drainage (storm sewers, inlets): 10-year return period; major storm drainage (regional facilities, detention ponds): 25- or 100-year return period; culverts under roads: 10- to 50-year depending on road classification (FHWA). Post-development peak flow for water quality control is often the 1- or 2-year storm per MS4 NPDES permit requirements. Always check your local stormwater management ordinance and state DOT design manual for the required return period by facility type.