Manning's n is a roughness coefficient used in Manning's equation to calculate open channel and pipe flow. Lower values indicate smoother surfaces with less resistance, while higher values indicate rougher surfaces. The tables below provide minimum, typical, and maximum values for design guidance.
Manning's Equation
US Customary (ft, cfs):
V = (1.486/n) R2/3 S1/2
SI Units (m, m/s):
V = (1/n) R2/3 S1/2
Where: V = velocity, n = Manning's roughness coefficient, R = hydraulic radius, S = slope
Showing 1-25 of 81 values
| Expand row | Material ▲ | Condition | n Min | n Typical | n Max | Category |
|---|---|---|---|---|---|---|
| Asphalt | Smooth | 0.013 | 0.015 | 0.016 | Lined | |
| Asphalt | Rough | 0.016 | 0.018 | 0.020 | Lined | |
| Brick | Glazed | 0.011 | 0.013 | 0.015 | Lined | |
| Brick | In cement mortar | 0.012 | 0.015 | 0.018 | Lined | |
| Cast Iron | Uncoated | 0.012 | 0.014 | 0.016 | Closed | |
| Cast Iron | Tuberculated (aged) | 0.015 | 0.020 | 0.035 | Closed | |
| Concrete | Precast, good joints | 0.011 | 0.013 | 0.015 | Closed | |
| Concrete | Precast, rough joints | 0.013 | 0.015 | 0.017 | Closed | |
| Concrete | Cast-in-place, steel forms | 0.012 | 0.013 | 0.014 | Closed | |
| Concrete | Cast-in-place, wood forms | 0.015 | 0.017 | 0.020 | Closed | |
| Concrete | Monolithic, smooth finish | 0.010 | 0.012 | 0.013 | Closed | |
| Concrete | Aged/deteriorated | 0.015 | 0.017 | 0.020 | Closed | |
| Concrete | Trowel finish | 0.011 | 0.013 | 0.015 | Lined | |
| Concrete | Float finish | 0.013 | 0.015 | 0.016 | Lined | |
| Concrete | Unfinished | 0.014 | 0.017 | 0.020 | Lined | |
| Concrete | Gunite, smooth | 0.016 | 0.019 | 0.023 | Lined | |
| Concrete | Gunite, wavy | 0.018 | 0.022 | 0.025 | Lined | |
| Concrete | On excavated rock | 0.017 | 0.020 | 0.023 | Lined | |
| Concrete Box | Smooth finish | 0.012 | 0.013 | 0.015 | Closed | |
| Concrete Box | Rough finish | 0.014 | 0.016 | 0.018 | Closed | |
| Corrugated Metal | 2-2/3 x 1/2 in corrugations, unpaved | 0.022 | 0.024 | 0.026 | Closed | |
| Corrugated Metal | 3 x 1 in corrugations, unpaved | 0.027 | 0.028 | 0.030 | Closed | |
| Corrugated Metal | 6 x 2 in corrugations (structural plate) | 0.033 | 0.035 | 0.037 | Closed | |
| Corrugated Metal | Paved invert (25% of circumference) | 0.018 | 0.021 | 0.023 | Closed | |
| Corrugated Metal | Paved invert (50% of circumference) | 0.015 | 0.018 | 0.020 | Closed |
Closed Conduits
Pipes, culverts, and box culverts
28 values
Lined Channels
Concrete, asphalt, riprap, grass, and other lined channels
22 values
Excavated Channels
Earth, gravel, and rock channels
12 values
Natural Streams
Minor streams, mountain streams, major streams, and floodplains
19 values
Design Guidance
Selecting n Values
- Use typical values for preliminary design
- Use maximum values when computing flood elevations or checking capacity
- Use minimum values when computing velocities for erosion analysis
- Consider future conditions (vegetation growth, sediment deposits, aging)
Composite n Values
For channels with varying roughness along the perimeter, calculate a composite n using Horton's equation or Einstein's method. Generally, use a weighted average based on wetted perimeter.
Riprap Estimation
For riprap-lined channels, Manning's n can be estimated using: n = 0.039 D501/6 where D50 is the median stone diameter in feet.
Primary Sources
- Chow, V.T. (1959). Open-Channel Hydraulics. McGraw-Hill, Table 5-6.
- FHWA HEC-22 (2009). Urban Drainage Design Manual, 3rd Ed. Tables 3-5, 3-6.
- FHWA HDS-4 (2012). Introduction to Highway Hydraulics, Table 5-1.
- USACE EM 1110-2-1601 (1994). Hydraulic Design of Flood Control Channels.
- FHWA HEC-15 (2005). Design of Roadside Channels with Flexible Linings.