Normal Depth Calculator (Manning's Equation)

Solve Manning's equation iteratively for uniform-flow normal depth in circular, rectangular, trapezoidal, and triangular sections, with velocity and flow-regime results.

Input Parameters

Channel Configuration

Select channel shape and enter geometry

Select the cross-section geometry

cfs

Target flow rate

Flow Parameters

Enter slope and roughness

ft/ft

Longitudinal bed slope (ft/ft or m/m)

Roughness coefficient

Channel Geometry

Enter dimensions for the selected shape

ft

Width at channel bottom

Normal Depth Overview

Normal depth is the depth at which uniform flow occurs in an open channel. At normal depth, the energy slope equals the channel bed slope, meaning gravitational forces are balanced by frictional resistance.

Manning's equation for uniform flow:

  • US Units: Q = (1.486/n) * A * R2/3 * S1/2
  • SI Units: Q = (1/n) * A * R2/3 * S1/2

This calculator iteratively solves for the depth that satisfies Manning's equation for the given discharge.

Typical Manning's n Values

Typical Manning roughness ranges by pipe or channel material
Materialn Value
Concrete pipe0.012 - 0.015
Corrugated metal pipe0.022 - 0.027
HDPE pipe0.010 - 0.012
Concrete channel0.013 - 0.017
Earth channel (clean)0.022 - 0.033
Grass-lined channel0.030 - 0.050

Source: Chow (1959), HEC-22 Table 7-1.

For educational purposes only. Not a substitute for professional engineering judgment.

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