Hydraulic Radius Calculator

About this calculator

This calculator finds the hydraulic radius, Rh = A/P, of an open channel or partially filled pipe — the cross-sectional area of flow (A) divided by the wetted perimeter (P), the length of the channel boundary actually in contact with the water. It's a key input to open-channel flow equations like Manning's equation, since a larger hydraulic radius means less frictional drag per unit of flow area and therefore faster, more efficient flow for the same slope and roughness.

Civil and hydraulic engineers use it to design canals, culverts, storm drains, and sewer pipes that carry the expected flow without overtopping or excessive friction losses; irrigation engineers use it to size channels for efficient water delivery; and students in fluid mechanics courses use it as a standard building block before applying Manning's or Chézy's equations. Enter the flow cross-sectional area and wetted perimeter to get the hydraulic radius.