Calculate stormwater runoff depth and volume using the NRCS SCS Curve Number method. Enter drainage area, rainfall depth, and curve number to get runoff in inches, acre-feet, and gallons. Includes CN reference table by land use and soil group.
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Shop Express Tools →The SCS/NRCS method estimates direct runoff from any rainfall event using the CN to characterize watershed response. The potential maximum retention (S) represents the maximum additional water the watershed can absorb after runoff begins. Initial abstraction (Ia = 0.2S) must be satisfied before runoff starts.
The SCS CN method is best for computing total storm runoff volume for detention pond sizing, BMP design, and comparing pre- vs. post-development runoff. For peak flow rate calculations, combine this with a unit hydrograph (SCS Type II or other distribution) or use the Rational Method (Q = CiA) for smaller watersheds. Many local jurisdictions specify which method is required — check your local stormwater manual.
NOAA Atlas 14 (hdsc.nws.noaa.gov/pfds) provides precipitation frequency estimates for any U.S. location. Select your state and coordinates to get rainfall depths for 2-year through 1000-year recurrence intervals at 5-minute through 60-day durations. For SCS method, use the 24-hour storm depth for the appropriate return period specified by your local regulations.
The SCS (Soil Conservation Service) Curve Number method, now called the NRCS method, is the most widely used rainfall-runoff method in stormwater engineering. Developed by the USDA, it estimates runoff depth from a rainfall event based on land use, soil type, and antecedent moisture conditions. The dimensionless Curve Number (CN) ranges from 0 (no runoff) to 100 (all rainfall becomes runoff). Reference: USDA Technical Release 55 (TR-55).
The CN depends on two factors: (1) land use and cover condition (impervious vs. pervious, vegetation density), and (2) hydrologic soil group (A through D). Group A soils (sand, gravel) have high infiltration; Group D soils (clay, poorly drained) have very low infiltration. Your county soil survey (USDA Web Soil Survey at websoilsurvey.nrcs.usda.gov) identifies soil groups. The table in the calculator provides standard CN values. For mixed land uses, use an area-weighted composite CN.
Group A: Sand, loamy sand, sandy loam — low runoff potential, high infiltration rate (>0.30 in/hr). Group B: Silt loam, loam — moderate infiltration (0.15–0.30 in/hr). Group C: Sandy clay loam — low infiltration, high runoff potential (0.05–0.15 in/hr). Group D: Clay loam, silty clay, clay — very low infiltration, highest runoff potential (<0.05 in/hr). Dual-class soils (B/D, C/D) exist where a restrictive layer affects drainage.
Initial abstraction (Ia) is the rainfall that must fall before any runoff occurs. It represents interception by vegetation, surface ponding, and infiltration before runoff begins. The SCS method assumes Ia = 0.2 × S, where S = (1000/CN) − 10. For CN = 75, S = 3.33 inches and Ia = 0.67 inches — meaning the first 0.67 inches of rain produces zero runoff. This is why light rain events on pervious surfaces often produce no significant runoff.
Design storm selection depends on project type and local regulations. Common practice: 2-year event for minor drainage systems; 10-year for main channels and cross drains; 25-year for major structures; 100-year for flood control design. Many jurisdictions also require a water quality storm (typically 0.75–1.5 inches) for detention and BMP sizing. Always check local stormwater management standards — requirements vary significantly by municipality and state.
Total watershed / drainage basin area
Design storm total depth (e.g. 24-hr storm)
From NRCS table below (1–100)
NRCS Curve Numbers by Land Use and Hydrologic Soil Group
| Land Use / Cover | Soil A | Soil B | Soil C | Soil D |
|---|---|---|---|---|
| Cultivated land — poor condition | 72 | 81 | 88 | 91 |
| Cultivated land — good condition | 62 | 71 | 78 | 81 |
| Pasture — poor condition | 68 | 79 | 86 | 89 |
| Pasture — good condition | 39 | 61 | 74 | 80 |
| Meadow — good condition | 30 | 58 | 71 | 78 |
| Woods — poor condition | 45 | 66 | 77 | 83 |
| Woods — good condition | 25 | 55 | 70 | 77 |
| Open space / lawns — good condition | 39 | 61 | 74 | 80 |
| Residential — 1/4-acre lots | 61 | 75 | 83 | 87 |
| Residential — 1/2-acre lots | 54 | 70 | 80 | 85 |
| Commercial / business (85% impervious) | 89 | 92 | 94 | 95 |
| Industrial (72% impervious) | 81 | 88 | 91 | 93 |
| Paved parking / roads / rooftops | 98 | 98 | 98 | 98 |
| Gravel roads / dirt roads | 76 | 85 | 89 | 91 |
Soil Group A: High infiltration (sand, gravel). B: Moderate. C: Low. D: Very low (clay, expansive soils, shallow water table). Source: USDA TR-55.