# Hydraulic diameter

## Description

For flow in a pipe or a sphere moving in a fluid the internal diameter is generally used today. Other shapes such as rectangular pipes or non-spherical objects have an equivalent diameter defined. For shapes such as squares, rectangular or annular ducts where the height and width are comparable, the characteristical dimension for internal flow situations is taken to be the hydraulic diameter. The hydraulic diameter can be calculated by the cross-sectional area and the wetted perimeter of the ducts. (The wetted perimeter for a channel is the total perimeter of all channel walls that are in contact with the flow. This means the length of the channel exposed to air is not included in the wetted perimeter).

Related formulas## Variables

D | Hydraulic diameter (m) |

A | The cross-sectional area (m^{2}) |

P | The wetted perimeter (the perimeter of the cross sectional area that is "wet") (m) |