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Manning Equation

Q = V. A = 
k / n
A R S½

Q: flow rate (ft3/s, in3/s, m3/s, cm3/s)
V: velocity (ft/s, in/s, m/s, cm/s)
A: cross-sectional area of flow (ft2, in2, m2, cm2)
k: for US Customary units: k=1.4859 ft/s; for SI units: k=1 m/s
n: Manning Coefficient (unitless)
R: hydraulic radius (ft, in, m, cm), R = A/P, where P is wetted perimeter
S: longitudinal slope of channel (ft/ft, in/in, m/m, cm/cm)

 Step 1: Select a System of Measurement:
  U.S. Customary Units:  ft & s
  U.S. Customary Units:  in & s
  SI Units:  m & s
  SI Units:  cm & s

 Step 2: Select cross-sectional shape of the conduit or natural channel:
  Rectangle or Trapezoid
  Triangle: V or right-angled

I do not claim in any way that these calculators are free of errors - I develop them as a hobby - so use them at your own discretion. If you encounter an error, please let me know here.