Manning's n Values Table — Sourced Reference

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) R^2/3 S^1/2

SI Units (m, m/s):

V = (1/n) R^2/3 S^1/2

Where: V = velocity, n = Manning's roughness coefficient, R = hydraulic radius, S = slope

FHWA source check (July 2026): HEC-22 Fourth Edition Table 9.1 lists only representative conduit ranges, including smooth concrete pipe at 0.010-0.011, smooth corrugated polyethylene at 0.009-0.015, and smooth PVC at 0.009-0.011. Those ranges were checked directly against the official FHWA manual; they are not a row-by-row provenance claim for the broader compiled table on this page. Field roughness can differ with abrasion, corrosion, deflection, and joint condition. See the HEC-22 Fourth Edition reference before using a table value in a storm-drain design.

Category:

Showing 1-25 of 81 values

Show:

					Category	Use value
Asphalt	Smooth	0.013	0.015	0.016	Lined	Open calculatorwith n = 0.015 for Asphalt
Asphalt	Rough	0.016	0.018	0.020	Lined	Open calculatorwith n = 0.018 for Asphalt
Brick	Glazed	0.011	0.013	0.015	Lined	Open calculatorwith n = 0.013 for Brick
Brick	In cement mortar	0.012	0.015	0.018	Lined	Open calculatorwith n = 0.015 for Brick
Cast Iron	Uncoated	0.012	0.014	0.016	Closed	Open calculatorwith n = 0.014 for Cast Iron
Cast Iron	Tuberculated (aged)	0.015	0.020	0.035	Closed	Open calculatorwith n = 0.020 for Cast Iron
Concrete	Precast, good joints	0.011	0.013	0.015	Closed	Open calculatorwith n = 0.013 for Concrete
Concrete	Precast, rough joints	0.013	0.015	0.017	Closed	Open calculatorwith n = 0.015 for Concrete
Concrete	Cast-in-place, steel forms	0.012	0.013	0.014	Closed	Open calculatorwith n = 0.013 for Concrete
Concrete	Cast-in-place, wood forms	0.015	0.017	0.020	Closed	Open calculatorwith n = 0.017 for Concrete
Concrete	Monolithic, smooth finish	0.010	0.012	0.013	Closed	Open calculatorwith n = 0.012 for Concrete
Concrete	Aged/deteriorated	0.015	0.017	0.020	Closed	Open calculatorwith n = 0.017 for Concrete
Concrete	Trowel finish	0.011	0.013	0.015	Lined	Open calculatorwith n = 0.013 for Concrete
Concrete	Float finish	0.013	0.015	0.016	Lined	Open calculatorwith n = 0.015 for Concrete
Concrete	Unfinished	0.014	0.017	0.020	Lined	Open calculatorwith n = 0.017 for Concrete
Concrete	Gunite, smooth	0.016	0.019	0.023	Lined	Open calculatorwith n = 0.019 for Concrete
Concrete	Gunite, wavy	0.018	0.022	0.025	Lined	Open calculatorwith n = 0.022 for Concrete
Concrete	On excavated rock	0.017	0.020	0.023	Lined	Open calculatorwith n = 0.020 for Concrete
Concrete Box	Smooth finish	0.012	0.013	0.015	Closed	Open calculatorwith n = 0.013 for Concrete Box
Concrete Box	Rough finish	0.014	0.016	0.018	Closed	Open calculatorwith n = 0.016 for Concrete Box
Corrugated Metal	2-2/3 x 1/2 in corrugations, unpaved	0.022	0.024	0.026	Closed	Open calculatorwith n = 0.024 for Corrugated Metal
Corrugated Metal	3 x 1 in corrugations, unpaved	0.027	0.028	0.030	Closed	Open calculatorwith n = 0.028 for Corrugated Metal
Corrugated Metal	6 x 2 in corrugations (structural plate)	0.033	0.035	0.037	Closed	Open calculatorwith n = 0.035 for Corrugated Metal
Corrugated Metal	Paved invert (25% of circumference)	0.018	0.021	0.023	Closed	Open calculatorwith n = 0.021 for Corrugated Metal
Corrugated Metal	Paved invert (50% of circumference)	0.015	0.018	0.020	Closed	Open calculatorwith n = 0.018 for Corrugated Metal

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 D₅₀^1/6 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 (2024). Urban Drainage Design Manual, 4th Ed., Table 9.1 (FHWA-HIF-24-006).
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).

Related Calculators & Guides

Manning's Pipe Flow Calculator Manning's Channel Calculator Manning's n Selection Guide