Calculate open-channel discharge, velocity, hydraulic radius, wetted perimeter, and flow area using Manning’s equation. Supports rectangular, trapezoidal, and circular channels in SI and US customary units.
A Manning flow calculator is a practical engineering tool for estimating gravity-driven flow in open channels. If you work in drainage design, culvert sizing, stormwater modeling, irrigation, or civil site development, this calculator helps you estimate discharge and velocity quickly from geometry, roughness, and slope. The calculation is based on Manning’s equation, one of the most widely used empirical relationships in hydraulics.
Manning’s equation predicts steady, uniform open-channel flow. It connects the channel’s roughness, hydraulic geometry, and slope to calculate discharge. In SI units, the equation is:
Q = (1/n) · A · R^(2/3) · S^(1/2)
In US customary units, a conversion constant is included:
Q = (1.486/n) · A · R^(2/3) · S^(1/2)
Where:
This page computes geometric properties from the selected channel shape, then applies Manning’s equation to output discharge and velocity. It currently supports:
After calculation, you get:
Select SI or US customary before entering values. The calculator automatically uses the correct Manning constant.
For rectangular channels, input bottom width and flow depth. For trapezoidal channels, input bottom width, depth, and side slope z as horizontal:vertical. For circular channels, input diameter and current flow depth.
The roughness coefficient represents resistance from boundary texture, vegetation, joints, irregularities, and other friction effects.
Use channel slope in decimal form, not percent. For example, 0.5% is entered as 0.005.
Getting n right is often more important than small geometry adjustments. Real systems can vary due to construction tolerances, aging, sediment, and vegetation. Start with references, then calibrate with observed conditions when possible.
| Channel Material / Condition | Typical n Range |
|---|---|
| Finished concrete | 0.011 – 0.015 |
| Rough concrete / shotcrete | 0.015 – 0.018 |
| Corrugated metal | 0.022 – 0.030 |
| Earth channel, clean and straight | 0.018 – 0.025 |
| Earth channel with grass or light weeds | 0.025 – 0.040 |
| Natural stream, irregular | 0.030 – 0.070+ |
For conservative design, engineers often test multiple roughness scenarios, including a “maintenance-deferred” case with higher n.
Given: width 2.0 m, depth 1.0 m, slope 0.001, n = 0.015.
Area A = 2.0 m². Wetted perimeter P = 4.0 m. Hydraulic radius R = 0.5 m.
Q ≈ (1/0.015) × 2.0 × 0.5^(2/3) × 0.001^(1/2) ≈ 2.66 m³/s. Velocity V = Q/A ≈ 1.33 m/s.
Given: bottom width 1.2 m, depth 0.6 m, side slope z = 2, slope 0.003, n = 0.03.
Use the calculator to derive A, P, and R from the trapezoidal geometry, then solve for Q and V. This workflow is useful during preliminary grading and erosion checks.
Given: diameter 1.0 m, depth 0.45 m, slope 0.002, n = 0.013. The calculator uses segment geometry to determine area and wetted perimeter at that depth before applying Manning’s equation.
In final design, Manning calculations are typically combined with hydraulic grade line checks, freeboard criteria, supercritical/subcritical regime review, and local code requirements.
Manning’s equation is empirical and best for turbulent, rough-boundary open-channel flow. It is not a full replacement for gradually varied flow analysis, backwater modeling, inlet control, outlet control, or pressurized pipe flow calculations. For complex networks, pair this calculator with hydraulic software and field verification.
Not for pressure-flow conditions. This tool is for open-channel or partially full gravity flow where a free surface exists.
Many designs use 0.012 to 0.015 for concrete, depending on finish and condition. Verify against your design standard.
For uniform flow approximations, yes—engineers often use bed/invert slope as energy slope. In non-uniform flow, they may differ.
In some geometries, wetted perimeter and hydraulic radius change nonlinearly with depth. Capacity does not always scale linearly.
This Manning flow calculator provides a fast, reliable way to estimate open-channel flow capacity and average velocity from channel geometry, roughness, and slope. It is ideal for preliminary hydraulic design, alternatives screening, and quick field checks. For high-stakes or regulatory projects, always validate assumptions, review governing standards, and confirm with detailed hydraulic analysis.