Rational Method Peak Runoff Calculator
Estimate a full-catchment peak runoff rate from entered area, coefficient and uniform rainfall intensity.
YOUR CALCULATION
Result
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Calculation steps
Work through one step at a time or reveal the complete calculation. The steps use your current inputs.
How to calculate
Multiply runoff coefficient, rainfall intensity and catchment area after SI conversion. Require duration at least the entered concentration time; calculate a separate constant-coefficient event water balance.
Q=CIA in SI; constant-coefficient event volume=C×rainfall depth×area
Worked example
At C=0.6, area 1 ha and intensity 60 mm/hour, the peak estimate is 0.1 m³/s. A 30-minute uniform event has modeled runoff volume 180 m³ when its duration meets the concentration-time condition.
Assumptions and input details
User-entered coefficient, intensity and time of concentration; no rainfall-frequency lookup or local design standard. Uniform rainfall across the entire catchment, no routing or storage. Rejects shorter storms for the full-area peak assumption. Event volume is a separate constant-coefficient water balance and is not the area of a simulated hydrograph. Not a flood-safety or drain-sizing certification.
References: Texas DOT: Rational Method assumptions and conversions
Using this calculator
Can I enter my own problem?
Yes. Replace the example values with your own inputs. The result, calculation steps, tables and applicable diagrams update from those inputs. Blank or invalid values clear the result; example values are never silently substituted for missing inputs.
Are the results exact?
Results use floating-point arithmetic and displayed rounding. Read the formula, assumptions and any stated data precision before interpreting the result.
Is my input uploaded?
The calculation runs in your browser. No account, file upload or external AI service is required. Inputs are not saved between page reloads.
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