Manning's n Values Table — Sourced Reference

Manning's n is the roughness coefficient in Manning's equation for open-channel and pipe flow: lower values mean smoother surfaces and less flow resistance. The tables below give minimum, typical and maximum values by material and condition for concrete, PVC, HDPE, steel and corrugated metal pipes, open channels and natural streams. Every row cites the published table or equation it comes from — 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, or use the open channel flow calculator for rectangular, trapezoidal, triangular, parabolic, or surveyed channels.

Designing a rock-lined channel or bank? Use the FHWA riprap sizing calculator to determine D50, gradation, and layer thickness before selecting a representative roughness value.

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 (September 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. HEC-22 notes they are laboratory values, so the concrete-pipe row on this page uses the HDS-4 design range (0.011-0.013) instead. 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 1-81 of 81 values

Show:

					Category	Use value
Asphalt	Smooth Chow (1959) Table 5-6	0.013	0.013	0.013	Lined	Open channel calculatorwith n = 0.013 for Asphalt
Asphalt	Rough Chow (1959) Table 5-6	0.016	0.016	0.016	Lined	Open channel calculatorwith n = 0.016 for Asphalt
Brick	Glazed Chow (1959) Table 5-6	0.011	0.013	0.015	Lined	Open channel calculatorwith n = 0.013 for Brick
Brick	In cement mortar Chow (1959) Table 5-6	0.012	0.015	0.018	Lined	Open channel calculatorwith n = 0.015 for Brick
Cast Iron	Uncoated Chow (1959) Table 5-6	0.011	0.014	0.016	Closed	Open pipe calculatorwith n = 0.014 for Cast Iron
Cast Iron	Coated Chow (1959) Table 5-6	0.010	0.013	0.014	Closed	Open pipe calculatorwith n = 0.013 for Cast Iron
Concrete	Precast pipe, good joints HDS-4 (2008) Table B.3	0.011	0.013	0.013	Closed	Open pipe calculatorwith n = 0.013 for Concrete
Concrete	Sewer with manholes, inlets, etc., straight Chow (1959) Table 5-6	0.013	0.015	0.017	Closed	Open pipe calculatorwith n = 0.015 for Concrete
Concrete	Cast-in-place, unfinished, steel forms Chow (1959) Table 5-6	0.012	0.013	0.014	Closed	Open pipe calculatorwith n = 0.013 for Concrete
Concrete	Cast-in-place, unfinished, rough wood forms Chow (1959) Table 5-6	0.015	0.017	0.020	Closed	Open pipe calculatorwith n = 0.017 for Concrete
Concrete	Finished (smooth) Chow (1959) Table 5-6	0.011	0.012	0.014	Closed	Open pipe calculatorwith n = 0.012 for Concrete
Concrete	Poor joints, deteriorated walls HDS-4 (2008) Table B.3 note	0.014	0.017	0.018	Closed	Open pipe calculatorwith n = 0.017 for Concrete
Concrete	Trowel finish Chow (1959) Table 5-6	0.011	0.013	0.015	Lined	Open channel calculatorwith n = 0.013 for Concrete
Concrete	Float finish Chow (1959) Table 5-6	0.013	0.015	0.016	Lined	Open channel calculatorwith n = 0.015 for Concrete
Concrete	Unfinished Chow (1959) Table 5-6	0.014	0.017	0.020	Lined	Open channel calculatorwith n = 0.017 for Concrete
Concrete	Gunite, good section Chow (1959) Table 5-6	0.016	0.019	0.023	Lined	Open channel calculatorwith n = 0.019 for Concrete
Concrete	Gunite, wavy section Chow (1959) Table 5-6	0.018	0.022	0.025	Lined	Open channel calculatorwith n = 0.022 for Concrete
Concrete	On good excavated rock Chow (1959) Table 5-6	0.017	0.020	0.020	Lined	Open channel calculatorwith n = 0.020 for Concrete
Concrete Box	Smooth finish HEC-22 (2024) Table 9.1	0.012	0.013	0.015	Closed	Open pipe calculatorwith n = 0.013 for Concrete Box
Concrete Box	Cast-in-place, rough wood forms HDS-4 (2008) Table B.3	0.015	0.016	0.017	Closed	Open pipe calculatorwith n = 0.016 for Concrete Box
Corrugated Metal	2-2/3 x 1/2 in corrugations, unpaved HDS-4 (2008) Table B.3	0.022	0.024	0.027	Closed	Open pipe calculatorwith n = 0.024 for Corrugated Metal
Corrugated Metal	3 x 1 in corrugations, unpaved HDS-4 (2008) Table B.3	0.027	0.028	0.028	Closed	Open pipe calculatorwith n = 0.028 for Corrugated Metal
Corrugated Metal	6 x 2 in corrugations (structural plate) HDS-4 (2008) Table B.3	0.033	0.035	0.035	Closed	Open pipe calculatorwith n = 0.035 for Corrugated Metal
Corrugated Metal	Paved invert (25% of perimeter), flowing full HDS-5 (2012) Eq. 3.8	0.020	0.022	0.024	Closed	Open pipe calculatorwith n = 0.022 for Corrugated Metal
Corrugated Metal	Paved invert (50% of perimeter), flowing full HDS-5 (2012) Eq. 3.8	0.018	0.019	0.021	Closed	Open pipe calculatorwith n = 0.019 for Corrugated Metal
Dragline-excavated	No vegetation Chow (1959) Table 5-6	0.025	0.028	0.033	Excavated/Dredged	Open channel calculatorwith n = 0.028 for Dragline-excavated
Dragline-excavated	Light brush on banks Chow (1959) Table 5-6	0.035	0.050	0.060	Excavated/Dredged	Open channel calculatorwith n = 0.050 for Dragline-excavated
Ductile Iron	Cement-mortar lined Chow (1959) Table 5-6	0.011	0.013	0.015	Closed	Open pipe calculatorwith n = 0.013 for Ductile Iron
Ductile Iron	Unlined Chow (1959) Table 5-6	0.011	0.014	0.016	Closed	Open pipe calculatorwith n = 0.014 for Ductile Iron
Earth	Straight and uniform, clean, recently completed Chow (1959) Table 5-6	0.016	0.018	0.020	Excavated/Dredged	Open channel calculatorwith n = 0.018 for Earth
Earth	Straight and uniform, short grass, few weeds Chow (1959) Table 5-6	0.022	0.027	0.033	Excavated/Dredged	Open channel calculatorwith n = 0.027 for Earth
Earth	Winding and sluggish, no vegetation Chow (1959) Table 5-6	0.023	0.025	0.030	Excavated/Dredged	Open channel calculatorwith n = 0.025 for Earth
Earth	Winding and sluggish, grass, some weeds Chow (1959) Table 5-6	0.025	0.030	0.033	Excavated/Dredged	Open channel calculatorwith n = 0.030 for Earth
Earth	Winding and sluggish, dense weeds or aquatic plants Chow (1959) Table 5-6	0.030	0.035	0.040	Excavated/Dredged	Open channel calculatorwith n = 0.035 for Earth
Earth	Stony bottom, weedy banks Chow (1959) Table 5-6	0.025	0.035	0.040	Excavated/Dredged	Open channel calculatorwith n = 0.035 for Earth
Earth	Cobble bottom, clean sides Chow (1959) Table 5-6	0.030	0.040	0.050	Excavated/Dredged	Open channel calculatorwith n = 0.040 for Earth
Floodplain	Pasture, short grass Chow (1959) Table 5-6	0.025	0.030	0.035	Natural	Open channel calculatorwith n = 0.030 for Floodplain
Floodplain	Pasture, high grass Chow (1959) Table 5-6	0.030	0.035	0.050	Natural	Open channel calculatorwith n = 0.035 for Floodplain
Floodplain	Cultivated, no crop Chow (1959) Table 5-6	0.020	0.030	0.040	Natural	Open channel calculatorwith n = 0.030 for Floodplain
Floodplain	Cultivated, mature field crops Chow (1959) Table 5-6	0.030	0.040	0.050	Natural	Open channel calculatorwith n = 0.040 for Floodplain
Floodplain	Scattered brush, heavy weeds Chow (1959) Table 5-6	0.035	0.050	0.070	Natural	Open channel calculatorwith n = 0.050 for Floodplain
Floodplain	Medium to dense brush, winter Chow (1959) Table 5-6	0.045	0.070	0.110	Natural	Open channel calculatorwith n = 0.070 for Floodplain
Floodplain	Medium to dense brush, summer Chow (1959) Table 5-6	0.070	0.100	0.160	Natural	Open channel calculatorwith n = 0.100 for Floodplain
Floodplain	Dense willows, summer Chow (1959) Table 5-6	0.110	0.150	0.200	Natural	Open channel calculatorwith n = 0.150 for Floodplain
Floodplain	Heavy timber, flood stage below branches Chow (1959) Table 5-6	0.080	0.100	0.120	Natural	Open channel calculatorwith n = 0.100 for Floodplain
Floodplain	Heavy timber, flood stage reaching branches Chow (1959) Table 5-6	0.100	0.120	0.160	Natural	Open channel calculatorwith n = 0.120 for Floodplain
Gabion	Mattress, rock D50 = 6 in, 1.6 ft deep HEC-15 (2005) Section 7.1, Table 2.2	0.069	0.069	0.069	Lined	Open channel calculatorwith n = 0.069 for Gabion
Grass	Class A retardance (36 in stems) HEC-15 (2005) Eq. 4.2, Table 4.4	0.096	0.167	0.241	Lined	Open channel calculatorwith n = 0.167 for Grass
Grass	Class B retardance (24 in stems) HEC-15 (2005) Eq. 4.2, Table 4.4	0.066	0.115	0.166	Lined	Open channel calculatorwith n = 0.115 for Grass
Grass	Class C retardance (8 in stems) HEC-15 (2005) Eq. 4.2, Table 4.4	0.035	0.061	0.088	Lined	Open channel calculatorwith n = 0.061 for Grass
Grass	Class D retardance (4 in stems) HEC-15 (2005) Eq. 4.2, Table 4.4	0.023	0.041	0.059	Lined	Open channel calculatorwith n = 0.041 for Grass
Grass	Class E retardance (1.6 in stems) HEC-15 (2005) Eq. 4.2, Table 4.4	0.015	0.026	0.037	Lined	Open channel calculatorwith n = 0.026 for Grass
Gravel	Uniform section, clean Chow (1959) Table 5-6	0.022	0.025	0.030	Excavated/Dredged	Open channel calculatorwith n = 0.025 for Gravel
HDPE	Corrugated exterior, smooth interior HEC-22 (2024) Table 9.1	0.009	0.012	0.015	Closed	Open pipe calculatorwith n = 0.012 for HDPE
HDPE	Corrugated interior HEC-22 (2024) Table 9.1	0.018	0.020	0.025	Closed	Open pipe calculatorwith n = 0.020 for HDPE
Major Stream	Regular section, no boulders Chow (1959) Table 5-6	0.025	0.030	0.060	Natural	Open channel calculatorwith n = 0.030 for Major Stream
Major Stream	Irregular, rough section Chow (1959) Table 5-6	0.035	0.045	0.100	Natural	Open channel calculatorwith n = 0.045 for Major Stream
Masonry	Rubble, cemented Chow (1959) Table 5-6	0.017	0.025	0.030	Lined	Open channel calculatorwith n = 0.025 for Masonry
Masonry	Dry rubble Chow (1959) Table 5-6	0.023	0.032	0.035	Lined	Open channel calculatorwith n = 0.032 for Masonry
Minor Stream	Clean, straight, no rifts or pools Chow (1959) Table 5-6	0.025	0.030	0.033	Natural	Open channel calculatorwith n = 0.030 for Minor Stream
Minor Stream	Clean, winding, some pools/shoals Chow (1959) Table 5-6	0.033	0.040	0.045	Natural	Open channel calculatorwith n = 0.040 for Minor Stream
Minor Stream	Winding, pools, some weeds and stones Chow (1959) Table 5-6	0.035	0.045	0.050	Natural	Open channel calculatorwith n = 0.045 for Minor Stream
Minor Stream	Sluggish, deep pools, weedy Chow (1959) Table 5-6	0.050	0.070	0.080	Natural	Open channel calculatorwith n = 0.070 for Minor Stream
Minor Stream	Very weedy, deep pools, floodway Chow (1959) Table 5-6	0.075	0.100	0.150	Natural	Open channel calculatorwith n = 0.100 for Minor Stream
Mountain Stream	Gravel, cobbles, few boulders Chow (1959) Table 5-6	0.030	0.040	0.050	Natural	Open channel calculatorwith n = 0.040 for Mountain Stream
Mountain Stream	Cobbles, large boulders Chow (1959) Table 5-6	0.040	0.050	0.070	Natural	Open channel calculatorwith n = 0.050 for Mountain Stream
PVC	Smooth interior HEC-22 (2024) Table 9.1	0.009	0.010	0.011	Closed	Open pipe calculatorwith n = 0.010 for PVC
Riprap	D50 = 6 in, small channel, 1.6 ft deep HEC-15 (2005) Table 2.2	0.069	0.069	0.069	Lined	Open channel calculatorwith n = 0.069 for Riprap
Riprap	D50 = 6 in, small channel, 3.3 ft deep HEC-15 (2005) Table 2.2	0.056	0.056	0.056	Lined	Open channel calculatorwith n = 0.056 for Riprap
Riprap	D50 = 12 in, small channel, 3.3 ft deep HEC-15 (2005) Table 2.2	0.080	0.080	0.080	Lined	Open channel calculatorwith n = 0.080 for Riprap
Riprap	D50 = 6 in, large channel, deep flow Anderson et al. (1970) Eq. 15	0.035	0.035	0.035	Lined	Open channel calculatorwith n = 0.035 for Riprap
Riprap	D50 = 12 in, large channel, deep flow Anderson et al. (1970) Eq. 15	0.040	0.040	0.040	Lined	Open channel calculatorwith n = 0.040 for Riprap
Riprap	D50 = 24 in, large channel, deep flow Anderson et al. (1970) Eq. 15	0.044	0.044	0.044	Lined	Open channel calculatorwith n = 0.044 for Riprap
Riprap	Grouted riprap HEC-15 (2005) Table 2.1	0.028	0.030	0.040	Lined	Open channel calculatorwith n = 0.030 for Riprap
Rock Cut	Smooth, uniform Chow (1959) Table 5-6	0.025	0.035	0.040	Excavated/Dredged	Open channel calculatorwith n = 0.035 for Rock Cut
Rock Cut	Jagged, irregular Chow (1959) Table 5-6	0.035	0.040	0.050	Excavated/Dredged	Open channel calculatorwith n = 0.040 for Rock Cut
Spiral Rib Metal	Smooth walls HDS-4 (2008) Table B.3	0.012	0.013	0.013	Closed	Open pipe calculatorwith n = 0.013 for Spiral Rib Metal
Steel	Riveted and spiral Chow (1959) Table 5-6	0.013	0.016	0.017	Closed	Open pipe calculatorwith n = 0.016 for Steel
Steel	Lockbar and welded Chow (1959) Table 5-6	0.010	0.012	0.014	Closed	Open pipe calculatorwith n = 0.012 for Steel
Vitrified Clay	Sewer pipe Chow (1959) Table 5-6	0.011	0.014	0.017	Closed	Open pipe calculatorwith n = 0.014 for Vitrified Clay
Vitrified Clay	Sewer with manholes, inlets, etc. Chow (1959) Table 5-6	0.013	0.015	0.017	Closed	Open pipe calculatorwith n = 0.015 for Vitrified Clay

Closed Conduits

Pipes, culverts, and box culverts

25 values

Lined Channels

Concrete, asphalt, riprap, grass, and other lined channels

25 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 table or equation each is drawn from. Use the typical value for design unless a code or review agency specifies otherwise. Where a source publishes only a range (HEC-22, HDS-4, and Chow's major-stream rows), Min and Max are that range and Typical is a representative value inside it.

Concrete Pipe

Condition	Min	Typical	Max	Source
Precast pipe, good joints	0.011	0.013	0.013	HDS-4 (2008) Table B.3
Straight sewer with manholes and inlets	0.013	0.015	0.017	Chow (1959) Table 5-6
Cast-in-place, steel forms	0.012	0.013	0.014	Chow (1959) Table 5-6
Finished (smooth)	0.011	0.012	0.014	Chow (1959) Table 5-6
Poor joints / deteriorated walls	0.014	0.017	0.018	HDS-4 (2008) Table B.3 note

Corrugated Metal Pipe (CMP)

Condition	Min	Typical	Max	Source
2-2/3 x 1/2 in corrugations, unpaved	0.022	0.024	0.027	HDS-4 (2008) Table B.3
3 x 1 in corrugations, unpaved	0.027	0.028	0.028	HDS-4 (2008) Table B.3
6 x 2 in structural plate	0.033	0.035	0.035	HDS-4 (2008) Table B.3
2-2/3 x 1/2 in, 25% paved invert (full flow)	0.020	0.022	0.024	HDS-5 (2012) Eq. 3.8
Spiral rib pipe, smooth walls	0.012	0.013	0.013	HDS-4 (2008) Table B.3

HDPE, PVC & Plastic Pipe

Material & condition	Min	Typical	Max	Source
PVC, smooth interior	0.009	0.010	0.011	HEC-22 (2024) Table 9.1
HDPE, dual-wall (corrugated ext., smooth int.)	0.009	0.012	0.015	HEC-22 (2024) Table 9.1
HDPE, single-wall corrugated interior	0.018	0.020	0.025	HEC-22 (2024) Table 9.1

Steel Pipe

Condition	Min	Typical	Max	Source
Lockbar and welded (smooth steel pipe)	0.010	0.012	0.014	Chow (1959) Table 5-6
Riveted and spiral	0.013	0.016	0.017	Chow (1959) Table 5-6

The widely cited n = 0.012 for steel pipe is Chow's lockbar-and-welded value. Use 0.016 for riveted or spiral steel pipe.

Open Channels (Lined & Excavated)

Lining / condition	Min	Typical	Max	Source
Concrete, trowel finish	0.011	0.013	0.015	Chow (1959) Table 5-6
Asphalt, smooth	0.013	0.013	0.013	Chow (1959) Table 5-6
Riprap, D50 = 6 in, small channel 1.6 ft deep	0.069	0.069	0.069	HEC-15 (2005) Table 2.2
Riprap, D50 = 6 in, large channel, deep flow	0.035	0.035	0.035	Anderson et al. (1970) Eq. 15
Earth, straight and uniform, clean	0.016	0.018	0.020	Chow (1959) Table 5-6
Grass, Class C retardance (8 in stems)	0.035	0.061	0.088	HEC-15 (2005) Eq. 4.2, Table 4.4

Riprap and grass n are not constants: riprap n falls as depth increases and grass n falls as shear increases. The grass row is HEC-15 Eq. 4.2 at 100 / 25 / 10 Pa (min / typical / max); iterate with your design shear.

Natural Streams & Floodplains

Condition	Min	Typical	Max	Source
Minor stream, clean, straight	0.025	0.030	0.033	Chow (1959) Table 5-6
Minor stream, winding, some pools	0.033	0.040	0.045	Chow (1959) Table 5-6
Major stream, regular section	0.025	0.030	0.060	Chow (1959) Table 5-6
Floodplain, pasture, short grass	0.025	0.030	0.035	Chow (1959) Table 5-6
Floodplain, medium to dense brush, summer	0.070	0.100	0.160	Chow (1959) Table 5-6

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 or conduits with varying roughness along the perimeter, calculate a composite n using Horton's equation or Einstein's method, which weight each material's n by its share of the wetted perimeter. FHWA HDS-5 Eq. 3.8 gives this form for culverts with a paved or lined invert: n_c = [Σ p_i n_i^1.5 / p]^2/3.

Riprap Estimation

Riprap n depends on flow depth. For roadside and other small channels, FHWA HEC-15 uses Eq. 6.1 (Blodgett, for average depth at least 1.5 D₅₀) or Eq. 6.2 (Bathurst, for shallower flow); its Table 2.2 gives 0.069 for D₅₀ = 6 in at 1.6 ft depth. For deep flow in large channels, Anderson et al. (1970) give n = 0.0395 D₅₀^1/6 with D₅₀ in feet, which is markedly lower. Gabion n uses the same relations with the basket rock's D₅₀ (HEC-15 Section 7.1).

Grass Linings

Grass n falls as the flow bends the stems. HEC-15 Eq. 4.2 gives n = C_n τ_o^-0.4 with τ_o in N/m² (0.213 C_n τ_o^-0.4 with τ_o in lb/ft²). Table 4.4 gives C_n = 0.605, 0.418, 0.220, 0.147 and 0.093 for retardance Classes A to E (stem heights 36, 24, 8, 4 and 1.6 in). Iterate with the design shear stress.

Where These Values Come From

Every row in the interactive table names the table or equation its values come from, under its condition; expand a row for the full reference and notes. Pipe values come from Chow (1959), FHWA HEC-22, FHWA HDS-4 and FHWA HDS-5. Open-channel and natural-stream values come from Chow (1959), FHWA HEC-15 and Anderson et al. (1970). HEC-22 and HDS-4 publish ranges only: for those rows Min and Max are the published range and Typical is a representative value inside it. Riprap and grass values depend on depth or shear and are tabulated at the conditions stated in each row.

Chow, V.T. (1959). Open-Channel Hydraulics. McGraw-Hill, Table 5-6.
FHWA HEC-22 (2024). Urban Drainage Design Manual, 4th Ed. (FHWA-HIF-24-006), Table 9.1. PDF
FHWA HDS-4 (2008). Introduction to Highway Hydraulics, 4th Ed. (FHWA-NHI-08-090), Appendix B, Table B.3. PDF
FHWA HDS-5 (2012). Hydraulic Design of Highway Culverts, 3rd Ed. (FHWA-HIF-12-026), Eq. 3.8 (composite roughness). PDF
FHWA HEC-15 (2005). Design of Roadside Channels with Flexible Linings, 3rd Ed. (FHWA-NHI-05-114), Tables 2.1, 2.2 and 4.4, Eq. 4.2 and Section 7.1. PDF
Anderson, A.G., Paintal, A.S., & Davenport, J.T. (1970). Tentative Design Procedure for Riprap-Lined Channels. NCHRP Report 108, Eq. 15. PDF

Frequently Asked Questions

What is Manning's n for concrete pipe?

Use n = 0.013 for precast concrete pipe with good joints: FHWA HDS-4 (2008) Table B.3 recommends 0.011-0.013 for design. HEC-22 Table 9.1 lists 0.010-0.011, but those are laboratory values. Chow (1959) gives 0.015 (range 0.013-0.017) for a straight concrete sewer with manholes and inlets, and HDS-4 notes that pipe with poor joints and deteriorated walls can reach 0.014-0.018.

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 within the FHWA HDS-4 (2008) Table B.3 range of 0.022-0.027; n varies with barrel size. 3 x 1 in corrugations run 0.027-0.028 and 6 x 2 in structural plate 0.033-0.035. For a paved invert, compute a composite n with HDS-5 Eq. 3.8: paving 25% of the perimeter of a 2-2/3 x 1/2 in pipe gives about 0.022 flowing full.

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). Dual-wall corrugated HDPE with a smooth interior is 0.009-0.015 (0.012 is a common design value), and single-wall HDPE with a corrugated interior is 0.018-0.025 (about 0.020). These are laboratory ranges; field values run higher with joints, deflection and deposits. Source: FHWA HEC-22 (2024) Table 9.1.

What is Manning's n for steel pipe (0.012)?

Smooth welded (lockbar and welded) steel pipe uses n = 0.012 typical (range 0.010-0.014), which is the widely cited 0.012 value for steel pipe. Riveted and spiral steel is rougher at 0.016 (range 0.013-0.017). Source: Chow (1959) Table 5-6; FHWA HDS-4 Table B.3 lists steel pipe at 0.009-0.013.

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 (good section) to 0.022 (wavy section). Source: Chow (1959) Table 5-6.

What Manning's n should I use for riprap or a grass-lined channel?

Neither is a constant. In a small channel, FHWA HEC-15 (2005) Table 2.2 gives 0.069 for 6 in (150 mm) riprap at 1.6 ft depth, 0.056 at 3.3 ft, and 0.080 for 12 in riprap at 3.3 ft. The familiar 0.035 and 0.040 come from Anderson et al. (1970), n = 0.0395 D50^(1/6), and apply only to deep flow in large channels. Grass n falls as shear bends the stems: HEC-15 Eq. 4.2 gives 0.061 for Class C grass at 25 Pa (0.5 lb/ft²) and 0.035 at 100 Pa.

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. Major streams (top width over 100 ft) are lower: 0.025-0.060 for a regular section. Source: Chow (1959) Table 5-6.

Related Calculators & Guides

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