Rainfall Runoff Calculator – Estimate Peak Flow, Runoff Depth and Volume
When rain falls on a catchment, part of it soaks into the ground, part is stored and part becomes surface runoff that flows into drains, channels or rivers. This Rainfall Runoff Calculator combines several classic hydrologic methods into one tool so you can estimate peak runoff, runoff depth and runoff volume for a range of scenarios.
The calculator supports both metric and US units and is designed for preliminary design, education and quick checks. For formal engineering design or regulatory submissions, always verify assumptions and calculations against local standards and professional judgment.
How the Rainfall Runoff Calculator is Organized
The tool has four modes that answer different hydrology questions:
- Rational Method: Peak runoff Q = C · i · A for small drainage areas.
- SCS Curve Number: Runoff depth and volume from rainfall depth and CN.
- Runoff Volume: Runoff volume from rainfall depth, effective C and area.
- Intensity → Runoff Depth: Convert rainfall intensity and duration into depth and effective runoff depth.
You can move between tabs using the same rainfall and catchment concepts, switching units where needed, to build a more complete picture of how a storm affects your site.
Mode 1 – Rational Method Peak Runoff (Q = C · i · A)
The Rational Method is one of the most widely used formulas for estimating peak discharge from small, mostly urban drainage areas. It states that peak runoff Q is proportional to rainfall intensity i, drainage area A and a runoff coefficient C that represents how impervious the catchment is.
In this tab you enter:
- Drainage area and its unit (acres, hectares, km² or mi²).
- Rainfall intensity and unit (mm/hr or in/hr).
- Runoff coefficient C.
- Decimal places for the output.
Internally, the calculator converts area and intensity into SI base units, computes Q in m³/s and then converts to cfs for US users. It also reports the converted area and intensity so you can see exactly what was used in the calculation.
Mode 2 – SCS Curve Number Runoff Depth and Volume
The SCS (now NRCS) Curve Number method estimates direct runoff depth from a storm using rainfall depth, a Curve Number and standardationships for potential retention and initial abstraction.
In this tab you enter:
- Rainfall depth and unit (mm or inches).
- Curve Number (typically between 30 and 98).
- Drainage area and unit.
- Decimal places for the results.
The calculator computes potential retention S, initial abstraction Ia ≈ 0.2 S and uses the SCS equation to estimate direct runoff depth. It then multiplies runoff depth by area to obtain runoff volume in cubic meters and also converts that volume to acre-feet for convenience.
Mode 3 – Runoff Volume from Rainfall Depth and C
Sometimes you simply need a quick estimate of how much runoff volume a storm will produce from a catchment. This tab treats the runoff coefficient as an effective factor that converts rainfall depth into runoff depth.
In this tab you specify:
- Rainfall depth and unit (mm or inches).
- Effective runoff coefficient C.
- Drainage area and unit.
- Decimal places.
The calculator reports rainfall depth, effective runoff depth, runoff volume in cubic meters, runoff volume in acre-feet and the same volume in liters. This is useful for sizing storage, detention or infiltration systems.
Mode 4 – Rainfall Intensity to Runoff Depth
Design storms and intensity–duration–frequency (IDF) curves are often defined in terms of rainfall intensity. To use them in runoff volume calculations or check whether a storm will exceed storage depth, you first need to convert intensity into rainfall depth.
In this mode you enter:
- Rainfall intensity (mm/hr or in/hr).
- Storm duration (minutes or hours).
- Optional runoff coefficient C.
- Decimal places.
The calculator multiplies intensity by duration to get rainfall depth, shows that depth in the chosen unit and the alternative unit (mm vs inches), and, if you supply a runoff coefficient, estimates an effective runoff depth.
Choosing Runoff Coefficients and Curve Numbers
Both the Rational Method and SCS Curve Number methody on parameters that represent how a catchment behaves during rainfall:
- Runoff coefficient C: Increases with imperviousness, slope and soil compaction.
- Curve Number (CN): Reflects land use, soil type and antecedent moisture.
Typical sources for these values include local design manuals, engineering textbooks or NRCS tables. Using appropriate values for your region and project type is essential for realistic results.
How to Use the Calculator Effectively
- Use the Rational Method tab for quick peak flow checks on small catchments.
- Switch to the SCS Curve Number tab to analyze direct runoff depth from a design storm.
- Use the Runoff Volume tab when sizing storage or detention based on a simple C factor.
- Use the Intensity → Runoff Depth tab together with IDF curves to convert intensities into depths.
Remember that all of these methods are simplified representations of real hydrologic behavior. They are most useful when combined with local guidelines, experience and, where necessary, more detailed modeling.
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Rainfall Runoff FAQs
Frequently Asked Questions Rainfall Runoff
Quick answers to common questions Rational Method, SCS Curve Number and runoff volume calculations.
The Rational Method is generally recommended for small urban catchments, often up to a few dozen or a few hundred acres, where runoff response isatively quick and uniform. Local design manuals usually specify the allowable range of drainage areas and storm durations for which the method is acceptable.
If rainfall depth is less than or equal to the initial abstraction (Ia), the SCS method predicts zero direct runoff because all rainfall is assumed to be lost to surface storage, interception and initial infiltration. The calculator automatically sets runoff depth to zero in this case.
The calculator uses standard conversion factors for area, depth, flow and volume (for example, acres to square meters and cfs to cubic meters per second). For most engineering purposes, the precision is more than adequate. If your project requires specific conversion factors, you can adjust the input or results accordingly.
That depends on your local regulations. Many agencies specify particular methods, parameters, safety factors and software packages for design. This tool is ideal for preliminary checks, education and “back-of-the-envelope” calculations, but final designs should always follow the rules in your jurisdiction.
Unusual results often come from inconsistent or unrealistic inputs, such as excessively high rainfall intensity, an inappropriate runoff coefficient, or a Curve Number outside the typical range for the site. Double-check your units, confirm that CN and C values match local tables and verify that the drainage area is entered correctly.