Manning’s n Table and Calculator
Look up Manning’s roughness coefficient by source, or build one for a natural channel with Cowan’s method.
Manning’s n values by source
Each source is shown under its own heading with its table number; values from different sources are never averaged together. “Use” opens a calculator with that n.
Chow’s Open-Channel Hydraulics (1959) is a copyrighted book, so only the rows that the U.S. Army Corps of Engineers reproduces in HEC-RAS Table 3-1 are shown; the full Chow table is cited, not copied. Pipe and culvert values come from FHWA and the TxDOT Hydraulic Design Manual, each under its own heading.
Build an n: Cowan’s method (USGS WSP 2339) or a composite section
Results
Enter your values; the answer appears here as you type.
- Cowan’s procedure
- n = (nb + n1 + n2 + n3 + n4) m: a base value for a straight, uniform channel in the bed material, plus adjustments for irregularity, variation in cross section, obstructions and vegetation, times a factor for meandering (USGS WSP 2339, Eq. 3).
- Flood plains
- The same sum with WSP 2339 Table 3: n2 = 0 and m = 1.0, and a vegetation range up to 0.200 for dense brush and timber (Eq. 6).
- Choosing within a range
- Picking a condition fills in the middle of its range. Change the number to the value you judge right; values outside the chosen range are kept and flagged.
- Composite section
- For one wetted perimeter made of several materials — a concrete bottom with grass banks, a lined invert in an earth channel — enter each part’s wetted length and n. Horton-Einstein, n = [Σ Pi ni1.5 / P]2/3, assumes each part carries the mean velocity of the section and is the form HEC-RAS uses to composite a main channel (Hydraulic Reference Manual, Ch. 2, eq. 2-6). Pavlovskii, n = [Σ Pi ni2 / P]1/2, adds the resisting forces; both are shown, the one you pick is used. A flood plain beside a channel is a different case: divide the section and let each panel keep its n (Channel Flow, n by segment).
- The n table
- Rows keep their source and table reference, and sources are never averaged. A source is shown only after two independent transcriptions agree and its rights are clear. HDS-5 culvert values are laboratory ranges; field values can be higher. In the calculators’ n lookup, a row with only a range fills the upper end of the range and says so.
The calculator is tested against the cases below every time its code changes.
| Case | What | Reference | Values compared | Largest difference | Result |
|---|---|---|---|---|---|
| The table loads with the calculator. The same results are in verification.json. | |||||
The Cowan sum is checked against the worked example in WSP 2339 Table 4 (tolerance ±0.0005). The composite-n equations are checked by hand calculations (±0.01%) — HEC-RAS publishes the equation without a worked example. Every row of the n table and the WSP 2339 tables is checked against a second, independent transcription.
- The vegetation-density method for wooded flood plains (WSP 2339) is not included.
- Composite n: Horton-Einstein and Pavlovskii only. Lotter’s conveyance form and the HEC-RAS rule for when to composite (main-channel side slopes steeper than 5H:1V) are not applied; you decide when a composite n is appropriate.
- Chow’s full table, including closed conduits, is cited but not reproduced (copyright); use the HDS-5 rows for pipes and culverts.
- Why do sources give different n values?
- They come from measurements under different conditions, and most tables give a range (minimum, normal, maximum). Choose within the range with judgment for the actual condition and maintenance of the surface; your local criteria may set the value.
- Why are the HEC-RAS values credited to Chow?
- HEC-RAS Table 3-1 reproduces values from Chow’s Open-Channel Hydraulics (1959). The table shows them as “Chow (1959) via HEC-RAS Table 3-1” so both the original source and the public reference are clear.
- When do I need a composite n?
- When one wetted perimeter has clearly different materials and you want a single n for a one-dimensional calculation, for example a trapezoidal channel with a concrete bottom and grassed side slopes. Enter each part’s wetted length and n; the composite is weighted by length. If the parts have very different depths, as a flood plain beside a channel, divide the section instead.
- When should I use Cowan’s method?
- For natural channels and floodplains that no table row describes well. USGS Water-Supply Paper 2339 builds n from a base value for the bed material plus adjustments for irregularity, changes in shape, obstructions and vegetation, multiplied by a factor for meandering.
- Can I attach the calculation sheet to a design report?
- You can print it and include it as a supporting calculation that you have checked. It is a calculation aid, not a sealed engineering document: the engineer of record reviews it and is responsible for the design, and the criteria of the reviewing agency decide which methods and values apply.
| Topic | Reference |
|---|---|
| Cowan’s procedure | Arcement, G.J., Jr., and Schneider, V.R. (1989). Guide for Selecting Manning’s Roughness Coefficients for Natural Channels and Flood Plains, USGS Water-Supply Paper 2339 (also FHWA-TS-84-204). |
| Manning’s n table | U.S. Army Corps of Engineers, Hydrologic Engineering Center. HEC-RAS 1D Hydraulic Reference Manual, Table 3-1 — from Chow, V.T. (1959), Open-Channel Hydraulics, McGraw-Hill. |
| Culverts and storm drain conduits | FHWA (2012). Hydraulic Design of Highway Culverts, HDS-5, 3rd ed., FHWA-HIF-12-026, Appendix B, Table B.1 (also HEC-22, 4th ed., Table 9.1). |
| Street and pavement gutters | FHWA (2024). Urban Drainage Design Manual, HEC-22, 4th ed., FHWA-HIF-24-006, Table 5.3. |
| Original method | Cowan, W.L. (1956). Estimating hydraulic roughness coefficients. Agricultural Engineering, 37(7). |
| Composite n | U.S. Army Corps of Engineers, Hydrologic Engineering Center. HEC-RAS 1D Hydraulic Reference Manual, Chapter 2, Composite Manning’s n for the Main Channel (eq. 2-6); Chow, V.T. (1959), Open-Channel Hydraulics (Horton 1933, Einstein 1934, Pavlovskii 1931). |
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A free calculation aid using the published methods above. Not a sealed engineering document; the engineer of record is responsible for design. HERO E&C · TBPELS Firm F-27793.
Full disclaimer
A free calculation aid. This calculator applies the published methods named on this page to the inputs you enter. It is not a sealed engineering document, has not been reviewed by a licensed professional for your project, and no warranty is expressed or implied. Check the results independently before using them for design, bidding or permitting.
The criteria of the reviewing agency decide which methods and values apply to a design. The engineer of record is responsible for the design.
Using this calculator does not create an engineer–client relationship. Calculations run in your browser; your inputs are not sent to or stored by HERO E&C. HERO E&C · TBPELS Firm F-27793.