Rational Method Calculator
Calculate peak stormwater flow using drainage subareas, weighted runoff coefficient and rainfall intensity.
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Calculate peak stormwater flow using drainage subareas, weighted runoff coefficient and rainfall intensity.
Open calculator →Check full-flow capacity, hydraulic radius and velocity for circular stormwater and sewer pipes.
Estimate flow capacity and velocity for common open-channel sections using Manning's equation.
Use the same engineering variables you already work with: areas, coefficients, slopes, diameters and intensities.
The tool applies the corresponding equation and exposes the key assumptions instead of hiding the logic.
Use the free result for a quick check or move to a professional toolkit when you need a more complete workflow.
The professional Stormwater Drainage Excel Toolkit adds Manning full-flow capacity, velocity checks, theoretical minimum diameter, PASS/FAIL checks and a printable project report.