Rainwater Runoff Calculator
Estimate stormwater runoff volume and peak flow rate using the Rational Method (Q = CiA) and the SCS Curve Number method.
Common values: Impervious=98, Lawns good condition=61, Woods=55, Pasture=68
Formulas Used
Rational Method
Q = C × i × A
- Q = Peak runoff rate (cfs — cubic feet per second)
- C = Dimensionless runoff coefficient (0–1)
- i = Rainfall intensity (inches/hour)
- A = Drainage area (acres)
- Volume = Q (cfs) × Duration (s) = cubic feet
SCS Curve Number Method (NRCS TR-55)
S = (1000 / CN) − 10
Ia = 0.2 × S
Q = (P − Ia)² / (P − Ia + S) [when P > Ia, else Q = 0]
- S = Potential maximum retention after runoff begins (inches)
- CN = SCS Curve Number (1–100)
- Ia = Initial abstraction — interception, infiltration before runoff (inches)
- P = Total rainfall depth = intensity × duration (inches)
- Q = Runoff depth (inches); Volume = Q × Area (acre-inches) ÷ 12 = acre-feet
Unit Conversions
- 1 cubic foot = 7.48052 US gallons
- 1 acre-foot = 43,560 cubic feet = 325,851 gallons
Assumptions & References
- The Rational Method assumes uniform rainfall intensity over the entire drainage area for a duration equal to the time of concentration. Best suited for areas < 200 acres.
- The SCS/NRCS Curve Number method uses the standard initial abstraction ratio Ia = 0.2S as defined in NRCS TR-55 (1986).
- Runoff coefficients (C) are typical mid-range values per ASCE and local stormwater design manuals; actual values vary with soil type, slope, and antecedent moisture.
- Common CN values: Impervious surfaces ≈ 98; Lawns (good condition) ≈ 61; Pasture (good) ≈ 61–68; Woods (good) ≈ 55; Row crops ≈ 72–75.
- This calculator does not account for time of concentration, antecedent moisture conditions (AMC I/II/III), or detention/retention storage.
- References: NRCS TR-55 (1986); ASCE Manual of Engineering Practice No. 36; Chow, Maidment & Mays, Applied Hydrology (1988).