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Flow Velocity in Straight Sewers Formula and Calculator

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Flow Velocity in Straight Sewers Formula and Calculator

Obstruction-free straight sewers will have a flow velocity of

Formula 1.0,
V = ( C / n ) · ( R2/3 · S1/2 )


V = Flow velocity ft/s
n = coefficient dependent on roughness of conduit surface
R = A / P - hydraulic radius, ft = area in ft2, of fluid divided by wetted perimeter, ft
S = energy loss, ft/ft of conduit length; approximately the slope of the conduit invert for uniform flow
C = 1.486 (conversion factor to account for change from metric units used in development of the formula)

The value of n can range from 0.013 in smooth concrete sewers to 0.02 in plain pipe. Further, n usually increases as a sewer pipe ages. Specific n values are given in engineering handbooks such as those cited in the references section of this webpage.

The water flow, cu ft/s is:

Q = A · V


A = cross-sectional area of flow, ft2.



  • Hicks - Handbook of Civil Engineering Calculations,
  • McGraw-Hill.
  • Kevin D. Wills, M.S.E., P.E., Consulting Engineer.