Hydraulic Radius Calculator
Pick a cross-section, enter its dimensions, and the calculator returns the flow area, wetted perimeter and hydraulic radius Rh = A / P used in open-channel and pipe flow.
How to find the hydraulic radius
- Choose the cross-section: full pipe, open channel or full duct.
- Enter the diameter, or the width and depth, in metres.
- Read the area, wetted perimeter and hydraulic radius.
Examples
Full round pipe
circular full, diameter = 0.2 m
Rh = 0.05 m (equals d/4)
Open rectangular channel
width = 2 m, depth = 1 m
area 2, perimeter 4, Rh = 0.5 m
Frequently asked questions
What is the hydraulic radius?
Hydraulic radius Rh is the flow area divided by the wetted perimeter, Rh = A / P. It captures how efficiently a cross-section carries flow.
Why is the hydraulic radius of a full pipe d/4?
Area is pi times (d/2) squared and the wetted perimeter is pi times d. Dividing area by perimeter cancels pi and leaves d divided by 4.
What is the wetted perimeter?
It is the length of the boundary in contact with the fluid. An open channel wets the bottom and sides, while a full duct wets all four walls.
How does an open channel differ from a full duct?
An open channel has a free surface, so the top is not wetted and the perimeter is width plus two depths. A full duct wets all four sides.
Where is hydraulic radius used?
It feeds open-channel formulas such as Manning's equation and the hydraulic diameter (four times Rh) used in pipe friction calculations.
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