SCS Curve Number Calculator
100 mm of rain falling on a catchment with a curve number of 75 produces 41.1371 mm of direct runoff.
Row crops on hydrologic soil group B about 75–80; forest 55–70; urban 80–98.
41.1371489136195 mm
Full answer, then cut to fewer decimals (not rounded):
- 41.1371489136195 mm
- 41.137148913619 mm
- 41.13714891361 mm
- 41.1371489136 mm
- 41.137148913 mm
- 41.13714891 mm
- 41.1371489 mm
- 41.137148 mm
- 41.13714 mm
- 41.1371 mm
- 41.137 mm
- 41.13 mm
- 41.1 mm
- Potential retention S
- 84.66666667 mm
- Initial abstraction Ia
- 16.93333333 mm
- Runoff coefficient Q/P
- 0.4113714891
- Runoff volume
- 20568.57446 m³
Step-by-step solution
- S = 25,400/CN − 254 = 84.66666667 mm; Ia = 0.2 S = 16.93333333 mm
- Q = (P − Ia)² / (P − Ia + S) = 83.06666667² ÷ 167.7333333 = 41.13714891 mm
SCS Curve Number Formula
Q = (P − Ia)² / (P − Ia + S)
S = 25,400/CN − 254 mm; Ia = λS (λ = 0.2)
Worked Example
100 mm of rain on a catchment with CN = 75:
- S = 25,400/CN − 254 = 84.66666667 mm; Ia = 0.2 S = 16.93333333 mm
- Q = (P − Ia)² / (P − Ia + S) = 83.06666667² ÷ 167.7333333 = 41.13714891 mm
Initial abstraction Ia: 16.93333333 mm · Runoff coefficient Q/P: 0.4113714891 · Runoff volume: 20568.57446 m³
GATE tip
In inches the same method uses S = 1000/CN − 10; do not mix the two forms.
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Frequently asked questions
What does CN depend on?
Hydrologic soil group, land use and cover, and antecedent moisture condition.
Why is there no runoff for small storms?
Rain below the initial abstraction Ia is held by interception, depression storage and early infiltration.
Sources
- The International System of Units (SI), 9th edition (SI Brochure), Bureau International des Poids et Mesures .
Last reviewed: . Factors are stored exactly and computed with 50-digit decimal arithmetic. See how we verify conversion factors.
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APA
Scientific Converters. (2026, October 15). SCS Curve Number Calculator. Retrieved October 29, 2026, from https://scientificconverters.com/calculators/scs-curve-number-calculator/
IEEE
Scientific Converters, "SCS Curve Number Calculator," scientificconverters.com. Accessed: Oct. 29, 2026. [Online]. Available: https://scientificconverters.com/calculators/scs-curve-number-calculator/