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Manning's Equation Calculator
Calculate flow velocity and discharge using Manning's equation for open channels and pipes. Includes roughness coefficient table.
Hydraulics & Water Resources Civil engineers, drainage designers Commercial Intent: HIGH
Engineering Formulas
Manning's Equation
V = (1/n) × R^(2/3) × S^(1/2)
Q = V × A
V: Flow velocity (m/s)
n: Manning's roughness coefficient
R: Hydraulic radius = A/P (m)
S: Channel bed slope (m/m)
Q: Discharge (m³/s)
A: Cross-sectional flow area (m²)
P: Wetted perimeter (m)
Engineering Notes
Manning's equation is empirical and best for turbulent flow in open channels.
For pressure pipes, use Darcy-Weisbach or Hazen-Williams instead.
Always verify that calculated velocity is within acceptable limits for scour and sedimentation.
Assumptions
• Steady, uniform flow conditions
• Gravity-driven flow (no pressure flow)
• Circular pipe assumed flowing full
• Trapezoidal channel uses 1:1 side slope
Common Mistakes
✕ Using wrong n value for pipe material
✕ Forgetting that pipe flow assumes full-flow conditions
✕ Not checking that flow depth does not exceed pipe diameter
Frequently Asked Questions
What is Manning's equation used for?
Manning's equation calculates flow velocity and discharge in open channels and pipes under gravity flow. It's used for drainage design, sewer sizing, canal design, and stormwater management.
What is Manning's n?
Manning's n is a roughness coefficient that represents the resistance to flow. Higher n values mean more resistance (slower flow). Concrete pipe has n ≈ 0.013, while natural channels can have n > 0.05.
References & Standards
Manning (1891)Chow (1959)
Manning (1891)
Original open channel flow formula
Chow (1959)
Open-Channel Hydraulics — definitive reference