Fluid Flow - Hydraulic Diameter

Calculate hydraulic diameter for various duct and pipe configurations. The hydraulic diameter is the characteristic length used in Reynolds Number calculations and the Darcy-Weisbach equation for pressure loss. This calculator supports circular, concentric circular, and rectangular geometries.

Calculator

Inputs

Geometry Type

Inner Diameter (d) (m)

Outer Radius (ro) (m)

Inner Radius (ri) (m)

Width (a) (m)

Height (b) (m)

Results

Hydraulic Diameter (dh)-- m

Configuration	Formula	Parameters
Circular Pipe/Duct	dh = d	d = inner diameter
Concentric Circular Tubes	dh = 2(ro - ri)	ro = outer radius, ri = inner radius
Rectangular Duct	dh = 2ab/(a+b)	a = width, b = height
Generic Formula	dh = 4A/p	A = cross-section area, p = wetted perimeter

Flow Classification

Laminar: Re < 2300
Transient: 2300 ≤ Re ≤ 4000
Turbulent: Re > 4000

Where Re (Reynolds Number) = (ρ × v × dh) / μ

The hydraulic diameter is used in the Darcy-Weisbach equation to calculate pressure loss in ducts and pipes.

Note: Hydraulic diameter is not the same as equivalent diameter. The equivalent diameter is the diameter of a circular duct that produces the same pressure loss as the rectangular duct.