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Rainwater Runoff Calculator

Estimate peak runoff with the Rational Method or calculate runoff volume from a rainfall event, including weighted composite catchments.

Preset note: Typical planning value — verify with local drainage criteria.
SurfaceAreaTypeCoefficient C
m²

Weighted C

0.9

Peak Flow

12.5 L/s

Peak Flow

0.0125 m³/s

Peak Flow

0.4414 cfs

Flow per area

0.0125 L/s/m²

Formula: Q (L/s) = C × i × A / 3600

Weighted C = Σ(CᵢAᵢ) / ΣAᵢ = 0.9

The Rational Method estimates peak flow and assumes the entered rainfall intensity corresponds to a duration related to catchment time of concentration. It is primarily a small-catchment planning method. Verify applicability, intensity and coefficients with local drainage criteria.

Peak Flow and Event Runoff Volume

Peak-flow mode applies the Rational Method to a user-supplied design rainfall intensity. Event-volume mode multiplies rainfall depth, catchment area and runoff coefficient. Composite surfaces use an area-weighted coefficient.

User-supplied rainfall

The tool does not infer rainfall from a location or unverified IDF data.

Composite catchment

Model roofs, paving, gravel and landscaped areas as separate schedule rows.

Transparent units

Review L/s, m³/s and cfs or rainfall, runoff and retained volume.

Runoff Coefficients and Limitations

Runoff coefficient depends on surface, slope, soil, antecedent moisture and local criteria. Presets are illustrative planning values only. For large or complex catchments, storage and routing effects require a more suitable hydrologic method.

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