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. Every value on this page is sourced to a standard reference — see where these values come from — so you can cite it in a design report or drainage submittal.
Need help picking a value for a specific situation? See the Manning's n selection guide, then run the numbers in the Manning's pipe flow calculator.
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
Common Manning's n Values (Sourced)
The most-requested values, with the minimum / typical / maximum range and the standard reference each is drawn from. Use the typical value for design unless a code or review agency specifies otherwise.
Concrete Pipe
| Condition | Min | Typical | Max | Source |
|---|---|---|---|---|
| Precast, good joints | 0.011 | 0.013 | 0.015 | Chow (1959) |
| Cast-in-place, steel forms | 0.012 | 0.013 | 0.014 | Chow (1959) |
| Monolithic, smooth finish | 0.010 | 0.012 | 0.013 | FHWA HEC-22 |
| Aged / deteriorated | 0.015 | 0.017 | 0.020 | USACE EM 1110-2-1601 |
Corrugated Metal Pipe (CMP)
| Condition | Min | Typical | Max | Source |
|---|---|---|---|---|
| 2-2/3 x 1/2 in corrugations, unpaved | 0.022 | 0.024 | 0.026 | FHWA HDS-4 |
| 3 x 1 in corrugations, unpaved | 0.027 | 0.028 | 0.030 | FHWA HDS-4 |
| 6 x 2 in structural plate | 0.033 | 0.035 | 0.037 | FHWA HDS-4 |
| Fully lined (smooth) | 0.012 | 0.013 | 0.015 | FHWA HDS-4 |
HDPE, PVC & Plastic Pipe
| Material & condition | Min | Typical | Max | Source |
|---|---|---|---|---|
| PVC, smooth interior | 0.009 | 0.010 | 0.011 | FHWA HEC-22 |
| HDPE, smooth interior | 0.009 | 0.011 | 0.012 | FHWA HEC-22 |
| HDPE, dual-wall (corrugated ext., smooth int.) | 0.010 | 0.012 | 0.013 | FHWA HEC-22 |
| HDPE, single-wall corrugated interior | 0.018 | 0.020 | 0.023 | FHWA HEC-22 |
Steel Pipe
| Condition | Min | Typical | Max | Source |
|---|---|---|---|---|
| Smooth, coated (commercial steel) | 0.010 | 0.012 | 0.014 | FHWA HEC-22 |
| Riveted and spiral | 0.013 | 0.015 | 0.017 | Chow (1959) |
The widely cited n = 0.012 for commercial steel pipe corresponds to smooth, coated steel. Use 0.015 for older riveted or spiral pipe.
Open Channels (Lined & Excavated)
| Lining / condition | Min | Typical | Max | Source |
|---|---|---|---|---|
| Concrete, trowel finish | 0.011 | 0.013 | 0.015 | Chow (1959) |
| Asphalt, smooth | 0.013 | 0.015 | 0.016 | Chow (1959) |
| Riprap, D50 = 6 in | 0.030 | 0.035 | 0.040 | FHWA HEC-15 |
| Earth, clean, straight, uniform | 0.016 | 0.020 | 0.023 | Chow (1959) |
| Short grass swale (Class C retardance) | 0.025 | 0.030 | 0.035 | FHWA HEC-15 |
Natural Streams & Floodplains
| Condition | Min | Typical | Max | Source |
|---|---|---|---|---|
| Minor stream, clean, straight | 0.025 | 0.030 | 0.033 | Chow (1959) |
| Minor stream, winding, some pools | 0.033 | 0.040 | 0.045 | Chow (1959) |
| Floodplain, pasture, short grass | 0.025 | 0.030 | 0.035 | Chow (1959) |
| Floodplain, heavy brush, summer | 0.070 | 0.100 | 0.160 | Chow (1959) |
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.
Where These Values Come From
The coefficients on this page are compiled from the standard hydraulic engineering references used across the profession — the same sources cited in agency design manuals. Pipe values (concrete, corrugated metal, PVC/HDPE, steel) come primarily from Chow (1959), FHWA HEC-22, and FHWA HDS-4. Open-channel and natural-stream values come from Chow (1959), USACE EM 1110-2-1601, and FHWA HEC-15. Each row in the interactive table above lists its specific source when expanded, so you can cite the exact reference in a design report.
- 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.
Frequently Asked Questions
What is Manning's n for concrete pipe?
Use n = 0.013 as the typical design value for precast concrete pipe with good joints, ranging from 0.011 (smooth monolithic finish) to 0.015 (rough joints). Cast-in-place with steel forms is about 0.013; aged or deteriorated pipe rises to 0.017-0.020. Source: Chow (1959) and FHWA HEC-22.
What is Manning's n for corrugated metal pipe (CMP)?
For standard 2-2/3 x 1/2 in (68 x 13 mm) corrugated metal pipe, use n = 0.024 typical (range 0.022-0.026). Larger 3 x 1 in corrugations run about 0.028, and 6 x 2 in structural plate about 0.035. A fully smooth-lined CMP drops to roughly 0.013. Source: FHWA HDS-4.
What Manning's n should I use for PVC or HDPE pipe?
For smooth-wall PVC use n = 0.010 (range 0.009-0.011). Smooth-wall HDPE is about 0.011, and dual-wall corrugated HDPE with a smooth liner is 0.012. Single-wall corrugated HDPE with a corrugated interior is much rougher at about 0.020. Source: FHWA HEC-22.
What is Manning's n for steel pipe (0.012)?
Smooth, coated steel pipe uses n = 0.012 typical (range 0.010-0.014), which is the widely cited 0.012 value for commercial steel pipe. Riveted and spiral steel is rougher at about 0.015. Source: FHWA HEC-22 for coated steel; Chow (1959) for riveted and spiral.
What Manning's n should I use for a concrete-lined channel?
For a trowel-finished concrete channel use n = 0.013 (range 0.011-0.015). A float finish is about 0.015, unfinished as-cast concrete about 0.017, and gunite 0.019-0.022 depending on smoothness. Source: Chow (1959) and USACE EM 1110-2-1601.
What is Manning's n for a natural stream or channel?
A clean, straight minor stream uses n = 0.030 typical (range 0.025-0.033). Winding streams with pools run 0.040-0.045, weedy sluggish reaches 0.070, and heavily vegetated floodplains can exceed 0.100. Source: Chow (1959).