The hydraulic diameter, , is commonly used when dealing with non-circular pipes, holes or ducts.
The definition of the hydraulic diamater is:
For a circular pipe or hole the hydraulic diamater is:
Where d is the real diameter of the pipe. Hence, for circular pipes the hydraulic diameter is the same as the real diameter of the pipe.
For a rectangular tube or hole with the width and the height the hydraulic diamter is:
Coaxial circular tube
For a coaxial circular tube with an inner diameter and an outer diameter the hydraulic diameter is: