Hydraulic Radius Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Fluid calculations are especially sensitive to density, viscosity, geometry, pressure conventions, and unit systems. Hydraulic Radius Calculator narrows that problem to the relationship used on this page, making the displayed hydraulic radius easier to audit against the inputs and the governing equation.

What this calculator does

The Hydraulic Radius Calculator uses Area, Wetted perimeter to estimate the page’s Hydraulic radius from the fluid-mechanics relationship below. It is meant for a defined geometry and property set, so the useful part is not just the headline number but also whether your density, viscosity, dimensions, pressure reference, and flow convention match the model.

How to use it

Start with the fields that actually drive this result: Area, Wetted perimeter. Keep units consistent with the menus beside the fields and avoid mixing values measured under different conditions. After calculating, change one input at a time if you are comparing scenarios; that makes cause-and-effect much easier to see.

How the calculation works

Hydraulic radius is Rh = A/Pw, where A is wetted flow area and Pw is wetted perimeter. It is a geometry measure used in open-channel and noncircular-flow resistance equations.

Example

Using the page’s default example (Area = 1 m²; Wetted perimeter = 2 m), the calculator reports Hydraulic radius of 0.5 m. Change one driving input at a time and confirm the result moves in the direction predicted by the equation; that is a quick way to catch a unit or mode mistake.

How to interpret the result

Interpret the Hydraulic radius within the fluid, geometry, pressure reference, and property values you entered. A numerically plausible answer can still be physically wrong if gauge/absolute pressure, diameter/radius, viscosity type, or unit convention is mismatched, so compare the result with the assumptions as well as the formula.

Limitations and notes

The current headline uses the entered wetted area and wetted perimeter; shape/radius selectors do not automatically reconstruct a full section. Make sure A and Pw describe the same water level and geometry.