Hydraulic Radius

The hydraulic radius measures how efficiently a fluid can move through a channel or conduit by comparing the flow area to the length of boundary touching the fluid. A larger ratio usually means less frictional resistance, because a smaller proportion of the flow is in contact with the walls. It is especially useful in pipe and channel calculations because many flow formulas depend on this geometry rather than on shape alone.

Use this formula whenever you need to characterize flow in an open channel, partly full pipe, culvert, or other conduit where wall contact affects resistance. It is commonly used in hydraulics, water resources, and civil engineering design calculations.

• Using the full pipe perimeter instead of only the wetted perimeter in contact with the fluid
• Mixing units so that area and perimeter are not both in consistent SI or imperial units
• Confusing hydraulic radius with pipe radius; they are equal only for a full circular pipe under specific conditions