Rainfall Runoff Calculator

What a Rainfall Runoff Calculator Does

A Rainfall Runoff Calculator estimates how much of a rainfall event becomes surface runoff, and how that runoff might behave as it moves through a catchment. In practical work, this is the foundation for sizing storm drains, designing culverts, planning detention storage, checking flooding risk, and understanding site drainage performance. Because real storms and real catchments are complex, many day-to-day designs rely on standard hydrologic shortcuts that are fast, transparent, and good enough for preliminary sizing.

This tool focuses on three widely used approaches. The Rational Method estimates peak flow (peak discharge) from rainfall intensity, drainage area, and a runoff coefficient. The Runoff Volume method estimates total runoff volume from rainfall depth, area, and a coefficient that represents losses. The NRCS Curve Number method estimates runoff depth from rainfall depth using a soil and land-cover parameter (CN), and then converts runoff depth into total volume using area. Together, these methods cover the most common “how much runoff” questions in site work, stormwater design, and hydrology planning.

Runoff Basics: Rainfall In, Losses Out, Runoff Remaining

When rain falls on a catchment, it does not all become runoff. Some water infiltrates into the ground, some is stored temporarily in surface depressions, some is intercepted by vegetation, and some evaporates. The portion that remains and connects to a drainage path becomes runoff. In highly impervious areas like paved lots and rooftops, losses are smaller and runoff forms quickly. In pervious areas with dry, sandy soil, losses are larger and runoff may be limited or delayed.

Two different outputs matter in design:

• Peak flow (Q): the maximum discharge rate during the event, important for pipes, channels, and culverts.
• Runoff volume (V): the total runoff produced, important for detention basins, storage tanks, and water harvesting.

Peak flow and runoff volume are related, but they are not the same. A short intense storm can create a high peak flow but a moderate volume. A long moderate storm can create a large volume but a lower peak. This is why the calculator separates “peak flow” and “volume” into dedicated modes.

Peak Flow with the Rational Method

The Rational Method is one of the most common ways to estimate peak runoff for small catchments. It is compact and intuitive: Q = C · i · A, where C is a runoff coefficient, i is rainfall intensity, and A is drainage area.

The most important input is rainfall intensity. In design practice, intensity should match a specific storm frequency (for example, a 10-year storm) and a duration similar to the catchment’s time of concentration. If you do not have an intensity value, you can approximate intensity from rainfall depth and duration as i = depth ÷ duration. This tool supports both direct intensity input and depth-plus-duration input.

The runoff coefficient C represents how quickly and how completely rainfall becomes runoff. Higher values indicate impervious or saturated conditions. Lower values indicate higher infiltration and surface storage. In real design work, C varies by land use, slope, soil, and drainage connectivity. The calculator is flexible: you enter the C that matches your assumption or local guidance.

Runoff Volume from Rainfall Depth and Runoff Coefficient

If your goal is storage sizing, a volume-based model is often more useful than a peak-flow-only model. A simple volume estimate multiplies the effective runoff depth by catchment area:

In the simplest version, effective rainfall depth is C × P, where P is rainfall depth and C is a runoff coefficient used as a loss factor. This is not a full infiltration model, but it is useful for quick planning. This tool also includes an optional initial loss depth, which is a practical way to represent early interception and depression storage that must be filled before runoff begins. When an initial loss is used, the effective rainfall depth becomes max(0, P − loss) × C.

Outputs are shown in multiple volume units: cubic meters, liters, and gallons. This is convenient when runoff estimates feed directly into tank sizing, drainage reports, or construction calculations.

NRCS Curve Number Runoff Depth and Volume

The NRCS Curve Number (CN) method estimates direct runoff depth from rainfall depth using a single parameter that represents land use and soil conditions. It is widely used because it provides a structured way to represent infiltration and storage without requiring fully distributed soil modeling. In this tool, you enter rainfall depth, a curve number, and the initial abstraction ratio (often assumed as 0.20 in simplified practice).

In the CN method, a retention parameter S is derived from CN, and the runoff depth Q is computed from rainfall depth P using a threshold and a nonlinear relationship. When rainfall is small relative to initial abstraction, runoff can be zero. As rainfall increases beyond that threshold, runoff rises faster.

This calculator reports the runoff depth in both mm and inches, then converts it to runoff volume based on your catchment area. It also reports an effective runoff fraction, which is the ratio of runoff depth to rainfall depth when rainfall is nonzero. That fraction helps you compare scenarios quickly.

Choosing Inputs That Make Sense

Area

Area is the multiplier that converts depth to volume and influences peak flow directly. If your catchment is large and heterogeneous, simple methods can become less reliable because rainfall and infiltration vary across the site. For small drainage areas, these methods are typically most useful.

Rainfall depth and intensity

Rainfall depth is total rainfall over the event. Intensity is the rate over time. Many design standards use intensity derived from local IDF curves for a chosen return period and duration. If you only have depth and duration, converting to average intensity is acceptable for preliminary work, but remember that real storms often have short bursts that exceed the average.

Runoff coefficient and curve number

C and CN both represent losses, but they do so in different ways. C is a direct multiplier on intensity or depth and is commonly used for peak flow planning. CN embeds nonlinear retention behavior and produces runoff depth from rainfall depth more explicitly. If your project requires alignment with a specific standard, choose the method that standard expects.

Why Scenario Tables Are Helpful

Design rarely depends on a single rainfall number. You may need to see how outcomes change from a 10 mm storm to an 80 mm storm, or from moderate intensity to intense convective rainfall. The Scenario Table tab builds a sequence of results across a range of rainfall depths or intensities using the same catchment assumptions. You can use the table to identify thresholds, compare sensitivity, and export the data to a spreadsheet for reporting, charting, or further engineering checks.

Common Interpretation Tips

• If peak flow is high, pipes and inlets are often the limiting design elements. Check conveyance capacity and surface overflow paths.
• If runoff volume is high, storage and outlet controls are often critical. Consider detention sizing and controlled discharge rates.
• If dew point, soil moisture, or season suggests wet conditions, runoff coefficients and curve numbers effectively increase compared to dry conditions.
• If your site has disconnected impervious areas draining to pervious zones, a single coefficient can overestimate runoff.

Limitations and Assumptions

These calculations are simplified representations of a complex physical process. Real runoff depends on rainfall pattern, spatial variability, infiltration capacity changes during a storm, microtopography, storage, slope, and drainage connectivity. Use this tool for planning, education, and preliminary design, and follow local stormwater manuals and engineering standards for final design and compliance.