How is it calculated?

Q = CiA/360 (Q m³/s, i mm/h, A ha). Valid for small catchments (< ~80 ha).

Peak Runoff Calculator (Rational Method)

Understanding the rational method peak runoff calculator — concept and principle

The rational method estimates the peak rate of storm runoff from a small catchment, such as a car park, housing estate, school site or road section. It is the most common starting point for sizing gutters, storm sewers, culverts and inlets, because it needs only three inputs: how much of the rain runs off, how hard it rains, and how large the area is.

Only the share C of the rain reaches the outlet; the rest soaks in or is held back.

How it is calculated

Q = C·i·A/360, with Q the peak flow (m³/s), C the dimensionless runoff coefficient, i the rainfall intensity (mm/h) and A the catchment area in hectares. The result is also shown in L/s (× 1000).

The constant 360 is pure unit conversion: 1 mm of rain on 1 ha is 10 m³, so 1 mm/h on 1 ha is 10 m³ per 3,600 s, or 1/360 m³/s.

The principle

If rain falls at a steady intensity for long enough, water from the most remote point eventually reaches the outlet, and from then on the whole catchment contributes at once. That moment, the time of concentration, gives the peak, which equals the rain rate times the area, reduced by C for losses to infiltration, depressions and evaporation. That is why the intensity must be read from an intensity–duration–frequency (IDF) curve for a storm duration equal to the time of concentration and for the chosen return period. Typical C values are about 0.7–0.95 for roofs and pavement and roughly 0.05–0.35 for lawns, depending on soil and slope; mixed areas use an area-weighted average.

Worked example

Inputs: C = 0.7, i = 60 mm/h, A = 5 ha.
C × i × A = 0.7 × 60 × 5 = 210.
Q = 210/360 = 0.5833 m³/s.
In litres: 0.5833 × 1000 = 583.3 L/s.

Each point is the calculator re-run with only “Runoff coefficient C” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Runoff coefficient C	Peak flow (m³/s)
0.175	0.1458333333
0.35	0.2916666667
0.525	0.4375
0.7	0.5833333333
0.875	0.7291666667
1.05	0.875
1.4	1.166666667

How to read the result

The value is the design peak for the chosen storm, not the total volume. To compare options, keep i and A fixed and change C: replacing lawn with pavement can triple or quadruple the peak, which is the basic reason for detention basins and permeable paving. A higher return period (say 10 instead of 5 years) raises i and therefore Q in direct proportion.

Common mistakes

Entering area in m² or km² instead of hectares (1 ha = 10,000 m²).
Using a 24-hour or daily rainfall average instead of the intensity for a duration equal to the time of concentration.
Applying it to large or mixed rural catchments where storage and timing matter.
Assuming C stays constant for very intense storms on saturated ground.

Keep in mind: The rational method suits small catchments, roughly under 80 ha, and gives a peak flow only. Drainage design must use your local design rainfall, return periods and standards, and should be checked by a qualified engineer.

Frequently asked questions

Where do I get rainfall intensity?

From the national or local IDF curves for your location, return period and duration.

How do I find the time of concentration?

Add overland and channel travel times, or use an empirical formula such as Kirpich for small rural basins.

Can I use it to size a detention pond?

Only roughly; storage design needs a runoff volume or hydrograph method.

Sources: T. J. Mulvaney, On the use of self-registering rain and flood gauges, Transactions of the Institution of Civil Engineers of Ireland, 1851 · V. T. Chow, D. R. Maidment and L. W. Mays, Applied Hydrology, McGraw-Hill, 1988

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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