The SCS Curve Number (CN) method estimates direct runoff from rainfall. Curve numbers range from 0 to 100, with higher values indicating greater runoff potential. The tables below provide CN values for AMC II (average antecedent moisture condition) by land use and Hydrologic Soil Group (HSG).
SCS Runoff Equation
Runoff (Q):
Q = (P - 0.2S)2 / (P + 0.8S)
for P > 0.2S, otherwise Q = 0
Maximum Retention (S):
S = (1000/CN) - 10 (inches)
Initial Abstraction (Ia):
Ia = 0.2S (traditional) or 0.05S (revised)
Where: Q = runoff depth, P = rainfall depth, S = maximum retention, CN = curve number
Quick CN table (AMC II)
Common TR-55 values by hydrologic soil group. Use the searchable table below for other covers and conditions.
| Land cover | HSG A | HSG B | HSG C | HSG D |
|---|---|---|---|---|
| Residential, 1/4-acre lots | 61 | 75 | 83 | 87 |
| Open space, good condition | 39 | 61 | 74 | 80 |
| Woods, good condition | 30 | 55 | 70 | 77 |
| Paved parking, roofs, driveways | 98 | 98 | 98 | 98 |
Source: USDA SCS TR-55, Tables 2-2a and 2-2c. Values shown are for average antecedent runoff condition (AMC II).
Know your CN? Send it directly to the runoff calculator, or use a row link in the table (HSG B by default).
Open curve number calculatorShowing 66 of 66 entries
| Land Use / Cover ▲ | HSG A | HSG B | HSG C | HSG D | % Imperv. | Calculate |
|---|---|---|---|---|---|---|
Brush (Fair condition) Brush - weed - grass mixture with brush the major element (50-75% ground cover) | 35 | 56 | 70 | 77 | - | Use CN 56 |
Brush (Good condition) Brush - weed - grass mixture with brush the major element (>75% ground cover) | 30 | 48 | 65 | 73 | - | Use CN 48 |
Brush (Poor condition) Brush - weed - grass mixture with brush the major element (<50% ground cover) | 48 | 67 | 77 | 83 | - | Use CN 67 |
Close-seeded Legumes - Contoured (Good) Close-seeded legumes or rotation meadow, contoured, good condition | 55 | 69 | 78 | 83 | - | Use CN 69 |
Close-seeded Legumes - Contoured (Poor) Close-seeded legumes or rotation meadow, contoured, poor condition | 64 | 75 | 83 | 85 | - | Use CN 75 |
Close-seeded Legumes - Straight Row (Good) Close-seeded or broadcast legumes or rotation meadow, straight rows, good condition | 58 | 72 | 81 | 85 | - | Use CN 72 |
Close-seeded Legumes - Straight Row (Poor) Close-seeded or broadcast legumes or rotation meadow, straight rows, poor condition | 66 | 77 | 85 | 89 | - | Use CN 77 |
Close-seeded Legumes - Terraced (Good) Close-seeded legumes or rotation meadow, terraced, good condition | 51 | 67 | 76 | 80 | - | Use CN 67 |
Close-seeded Legumes - Terraced (Poor) Close-seeded legumes or rotation meadow, terraced, poor condition | 63 | 73 | 80 | 83 | - | Use CN 73 |
Commercial/Business (72% impervious) Urban commercial districts with moderate impervious cover | 81 | 88 | 91 | 93 | 72% | Use CN 88 |
Commercial/Business (85% impervious) Urban commercial districts with high impervious cover | 89 | 92 | 94 | 95 | 85% | Use CN 92 |
Dirt Roads Unpaved dirt roads | 72 | 82 | 87 | 89 | - | Use CN 82 |
Fallow - Bare Soil Fallow land with bare soil, no crop residue | 77 | 86 | 91 | 94 | - | Use CN 86 |
Fallow - Crop Residue (Good) Fallow land with crop residue cover, good hydrologic condition (>20% residue cover) | 74 | 83 | 88 | 90 | - | Use CN 83 |
Fallow - Crop Residue (Poor) Fallow land with crop residue cover, poor hydrologic condition (<20% residue cover) | 76 | 85 | 90 | 93 | - | Use CN 85 |
Farmsteads Buildings, lanes, driveways, and surrounding lots on farms | 59 | 74 | 82 | 86 | - | Use CN 74 |
Gravel Roads Unpaved gravel roads and parking areas | 76 | 85 | 89 | 91 | - | Use CN 85 |
Industrial (72% impervious) Industrial districts with typical impervious cover | 81 | 88 | 91 | 93 | 72% | Use CN 88 |
Meadow (Continuous grass) Meadow - continuous grass, protected from grazing, generally mowed for hay | 30 | 58 | 71 | 78 | - | Use CN 58 |
Newly Graded Areas Newly graded areas with exposed soil (pervious areas only, no vegetation) | 77 | 86 | 91 | 94 | - | Use CN 86 |
Open Space (Fair condition) Open space, parks, golf courses - grass cover 50-75% | 49 | 69 | 79 | 84 | - | Use CN 69 |
Open Space (Good condition) Open space, parks, golf courses - grass cover >75% | 39 | 61 | 74 | 80 | - | Use CN 61 |
Open Space (Poor condition) Open space, parks, golf courses - grass cover <50% | 68 | 79 | 86 | 89 | - | Use CN 79 |
Pasture - Contoured (Fair) Pasture with contoured grazing management, fair condition | 25 | 59 | 75 | 83 | - | Use CN 59 |
Pasture - Contoured (Good) Pasture with contoured grazing management, good condition | 6 | 35 | 70 | 79 | - | Use CN 35 |
Pasture - Contoured (Poor) Pasture with contoured grazing management, poor condition | 47 | 67 | 81 | 88 | - | Use CN 67 |
Pasture/Range (Fair condition) Pasture, grassland, or range - not heavily grazed (50-75% ground cover) | 49 | 69 | 79 | 84 | - | Use CN 69 |
Pasture/Range (Good condition) Pasture, grassland, or range - lightly grazed (>75% ground cover) | 39 | 61 | 74 | 80 | - | Use CN 61 |
Pasture/Range (Poor condition) Pasture, grassland, or range - heavily grazed with no mulch (<50% ground cover) | 68 | 79 | 86 | 89 | - | Use CN 79 |
Paved Parking Lots, Roofs, Driveways Impervious surfaces including asphalt, concrete, roofs | 98 | 98 | 98 | 98 | 100% | Use CN 98 |
Paved Roads with Curbs and Storm Sewers Streets and roads with curbs and storm sewers | 98 | 98 | 98 | 98 | 100% | Use CN 98 |
Paved Roads with Open Ditches Streets and roads with open ditches | 83 | 89 | 92 | 93 | - | Use CN 89 |
Residential (1 acre lots) Low density residential, lot size 1 acre, 20% impervious | 51 | 68 | 79 | 84 | 20% | Use CN 68 |
Residential (1/2 acre lots) Medium-low density residential, lot size 1/2 acre, 25% impervious | 54 | 70 | 80 | 85 | 25% | Use CN 70 |
Residential (1/3 acre lots) Medium density residential, lot size 1/3 acre, 30% impervious | 57 | 72 | 81 | 86 | 30% | Use CN 72 |
Residential (1/4 acre lots) Medium-high density residential, lot size 1/4 acre, 38% impervious | 61 | 75 | 83 | 87 | 38% | Use CN 75 |
Residential (1/8 acre lots) High density residential, lot size 1/8 acre or less, 65% impervious | 77 | 85 | 90 | 92 | 65% | Use CN 85 |
Residential (2 acre lots) Very low density residential, lot size 2 acres, 12% impervious | 46 | 65 | 77 | 82 | 12% | Use CN 65 |
Roofs (Impervious) Building rooftops - impervious surfaces | 98 | 98 | 98 | 98 | 100% | Use CN 98 |
Row Crops - Contoured & Terraced (Good) Row crops contoured and terraced, good hydrologic condition | 62 | 71 | 78 | 81 | - | Use CN 71 |
Row Crops - Contoured & Terraced (Poor) Row crops contoured and terraced, poor hydrologic condition | 66 | 74 | 80 | 82 | - | Use CN 74 |
Row Crops - Contoured (Good) Row crops planted on contour, good hydrologic condition | 65 | 75 | 82 | 86 | - | Use CN 75 |
Row Crops - Contoured (Poor) Row crops planted on contour, poor hydrologic condition | 70 | 79 | 84 | 88 | - | Use CN 79 |
Row Crops - Contoured + Residue (Good) Row crops with residue cover, contoured, good condition | 64 | 74 | 81 | 85 | - | Use CN 74 |
Row Crops - Contoured + Residue (Poor) Row crops with residue cover, contoured, poor condition | 69 | 78 | 83 | 87 | - | Use CN 78 |
Row Crops - Straight Row (Good) Row crops planted in straight rows, good hydrologic condition | 67 | 78 | 85 | 89 | - | Use CN 78 |
Row Crops - Straight Row (Poor) Row crops planted in straight rows, poor hydrologic condition | 72 | 81 | 88 | 91 | - | Use CN 81 |
Row Crops - Straight Row + Residue (Good) Row crops with crop residue cover, straight rows, good condition | 64 | 75 | 82 | 85 | - | Use CN 75 |
Row Crops - Straight Row + Residue (Poor) Row crops with crop residue cover, straight rows, poor condition | 71 | 80 | 87 | 90 | - | Use CN 80 |
Row Crops - Terraced + Residue (Good) Row crops with residue, contoured and terraced, good condition | 61 | 70 | 77 | 80 | - | Use CN 70 |
Row Crops - Terraced + Residue (Poor) Row crops with residue, contoured and terraced, poor condition | 65 | 73 | 79 | 81 | - | Use CN 73 |
Small Grain - Contoured & Terraced (Good) Small grain contoured and terraced, good hydrologic condition | 59 | 70 | 78 | 81 | - | Use CN 70 |
Small Grain - Contoured & Terraced (Poor) Small grain contoured and terraced, poor hydrologic condition | 61 | 72 | 79 | 82 | - | Use CN 72 |
Small Grain - Contoured (Good) Small grain planted on contour, good hydrologic condition | 61 | 73 | 81 | 84 | - | Use CN 73 |
Small Grain - Contoured (Poor) Small grain planted on contour, poor hydrologic condition | 63 | 74 | 82 | 85 | - | Use CN 74 |
Small Grain - Contoured + Residue (Good) Small grain with residue, contoured, good condition | 60 | 72 | 80 | 83 | - | Use CN 72 |
Small Grain - Contoured + Residue (Poor) Small grain with residue, contoured, poor condition | 62 | 73 | 81 | 84 | - | Use CN 73 |
Small Grain - Straight Row (Good) Small grain crops in straight rows, good hydrologic condition | 63 | 75 | 83 | 87 | - | Use CN 75 |
Small Grain - Straight Row (Poor) Small grain crops in straight rows, poor hydrologic condition | 65 | 76 | 84 | 88 | - | Use CN 76 |
Small Grain - Straight Row + Residue (Good) Small grain with residue cover, straight rows, good condition | 60 | 72 | 80 | 84 | - | Use CN 72 |
Small Grain - Straight Row + Residue (Poor) Small grain with residue cover, straight rows, poor condition | 64 | 75 | 83 | 86 | - | Use CN 75 |
Small Grain - Terraced + Residue (Good) Small grain with residue, terraced, good condition | 58 | 69 | 77 | 80 | - | Use CN 69 |
Small Grain - Terraced + Residue (Poor) Small grain with residue, terraced, poor condition | 60 | 71 | 78 | 81 | - | Use CN 71 |
Woods (Fair condition) Woods are grazed but not burned, some forest litter covers the soil | 36 | 60 | 73 | 79 | - | Use CN 60 |
Woods (Good condition) Woods are protected from grazing, litter and brush adequately cover the soil | 30 | 55 | 70 | 77 | - | Use CN 55 |
Woods (Poor condition) Forest litter, small trees, and brush are destroyed by heavy grazing or regular burning | 45 | 66 | 77 | 83 | - | Use CN 66 |
Hydrologic Soil Groups (HSG)
High Infiltration
Infiltration: 0.3-0.45 in/hr
Typical: Sand, Loamy sand, Sandy loam
Moderate Infiltration
Infiltration: 0.15-0.3 in/hr
Typical: Silt loam, Loam
Slow Infiltration
Infiltration: 0.05-0.15 in/hr
Typical: Sandy clay loam, Clay loam, Silty clay loam
Very Slow Infiltration
Infiltration: 0-0.05 in/hr
Typical: Clay, Silty clay, Sandy clay
Design Guidance
Antecedent Moisture Condition (AMC)
- AMC I (Dry): Use when soils are dry (5-day antecedent rainfall < 0.5" dormant, < 1.4" growing)
- AMC II (Average): Standard design condition (table values)
- AMC III (Wet): Use when soils are saturated (5-day antecedent rainfall > 1.1" dormant, > 2.1" growing)
Composite Curve Number
For watersheds with multiple land uses, calculate an area-weighted composite CN:
CNcomposite = sum(CNi x Ai) / Atotal
Dual HSG Soils
Soils with dual classification (e.g., A/D, B/D, C/D) behave as the first letter when adequately drained and the second letter when undrained. Check local soil surveys for drainage status.
Urban Development
For urban areas, consider the connected impervious area versus total impervious area. TR-55 provides guidance on adjusting CN for unconnected impervious areas that drain to pervious surfaces.
Frequently Asked Questions
What curve number should I use for residential lots?
For 1/4-acre residential lots (38% impervious), the SCS curve number is 61 (HSG A), 75 (B), 83 (C), and 87 (D) at AMC II (TR-55 Table 2-2a). Denser 1/8-acre lots run higher (77–92) and larger 1-acre lots run lower (51–84). Pick the row matching your lot size and soil group.
What is the curve number for impervious surfaces?
Paved parking lots, roads with curbs and storm sewers, and roofs use a curve number of 98 for all hydrologic soil groups (TR-55 Table 2-2a). Paved roads with open ditches are lower (83–93), and gravel roads are 76–91 depending on soil group.
How do I calculate a composite curve number?
For a watershed with multiple land uses, area-weight the curve numbers: CN_composite = Σ(CNᵢ × Aᵢ) / A_total. Look up each sub-area’s CN by land use and soil group, multiply by its area, sum, and divide by total area. The curve number calculator does this automatically.
What antecedent moisture condition do the table values use?
All values in the table are for AMC II (average moisture). For dry conditions use AMC I (lower CN, less runoff) and for saturated conditions use AMC III (higher CN, more runoff); the table’s AMC adjuster converts an AMC II value to AMC I or III.
Primary Sources
- USDA SCS TR-55 (1986). Urban Hydrology for Small Watersheds. Tables 2-2a, 2-2b, 2-2c.
- USDA NRCS NEH Part 630 (2004). National Engineering Handbook, Chapter 9 - Hydrologic Soil-Cover Complexes.
- USDA NRCS NEH Part 630 (2004). Chapter 7 - Hydrologic Soil Groups.
- USDA NRCS NEH Part 630 (2004). Chapter 10 - Estimation of Direct Runoff from Storm Rainfall.
Related Calculators & Guides
Ready to compute runoff? Use the curve number calculator to find a composite CN and runoff depth, or read the SCS curve number method explained for a step-by-step walkthrough.