Hydraulic Diameter Calculator

Flow passage shape *

Pick a standard shape or enter area and wetted perimeter.

Input length unit *

Used for all dimension and perimeter inputs.

Output unit

Hydraulic diameter will be shown in this unit.

Width, w *

For a duct with four wetted sides.

Height, h *

Diameter, d *

For a full circular conduit, D_h equals d.

Outer diameter, D_o *

Inner diameter, D_i *

For annulus, D_h simplifies to D_o − D_i.

Side length, a *

For a square, D_h equals a.

Side a *

Side b *

Side c *

Area uses Heron's formula; perimeter is a+b+c.

Radius, r *

Assumes the flat diameter is also wetted.

Semi-major axis, a *

Semi-minor axis, b *

Perimeter uses a common Ramanujan approximation.

Area, A *

Area unit *

Wetted perimeter, P_w *

Use the same length unit selected above.

Formula Used

Hydraulic diameter converts a non-circular passage into an equivalent diameter for flow correlations such as friction factor and Reynolds number.

Core relation

D_h = 4A / P_w

Tip: If your geometry is unusual, use Custom and supply the true area and wetted perimeter directly.

Case	Shape	Inputs	Hydraulic Diameter
1	Circular	d = 50 mm	D_h = 50 mm
2	Rectangular	w = 200 mm, h = 100 mm	D_h ≈ 133.333 mm
3	Annulus	D_o = 100 mm, D_i = 60 mm	D_h = 40 mm
4	Custom	A = 0.015 m², P_w = 0.80 m	D_h = 0.075 m