Manning’s Open Channel Flow Calculator
Four shapes including the partly full pipe most tools get wrong, the reverse solve for normal depth, the Froude number — and the fact that a pipe carries most at 0.938 of its diameter, not full.
The channel
Internal, not nominal
m
m
Horizontal per 1 vertical — 2 means 2H:1V
H:1V
Water depth, not channel depth
m
Advanced / Custom Input — the section
The conversion factor k is 1.0 in SI and 1.4859 in US customary.
For the freeboard check. 0 to skip.
m
What it is lined with
Sets Manning’s n
Filled in from the surface — override with your own
Advanced / Custom Input — roughness
Nudges n within the tabulated range for that surface.
Slope, and what you want to find
Fall per unit length — 0.005 is 0.5%, or 1 in 200
m/m
Most tools only do the first one
The tool will find the depth this produces
m³/s
Advanced / Custom Input — the slope
Used when the format above is not the decimal one.
Design checks
Below this, solids settle out
m/s
Above this, the lining erodes
m/s
Advanced / Custom Input — the checks
Circular pipes. Above about 80% treat the result as a capacity warning.
%
Checked against the total channel depth if you entered one.
m
Result
Pick a shape, give it a size and a slope, then press “Work out the flow”.
Estimate only — verify with a qualified civil engineer and your local drainage authority before final design or construction.