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Required Flow Velocity in Sewers Formula and Calculator

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Required Flow Velocity in Sewers Formula and Calculator

To ensure full carriage of the solids, soils, refuse, and sand conveyed by storm and sanitary sewers, minimum flow velocities must be maintained. For storm sewers, the minimum flow velocity should be 3 ft/s; for sanitary sewers, 2 ft/s is recommended. Full-flow velocity of the liquid in a sewer is given by:

Formula 1,
V = ( 0.59 / n ) · d2/3 · S1/2


V = Flow velocity ft/s,
Q = Flow quantity, ft3 / s
n = coefficient dependent on roughness of conduit surface,
S = energy loss, ft/ft of conduit length; approximately the slope of the conduit invert for uniform flow,
d = inside diameter of pipe, ft.

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 quantity, cu ft/s for a full-flowing sewer pipe is given by:

Formula 2,
Q = ( 0.463 / n ) · ( d8/3 · S1/2 )



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