Equivalent and Hydraulic Diameter Calculator

Calculate hydraulic diameter for rectangular and noncircular flow passages.

Dh = 4A / Pw

Duct width a (m)

Duct height b (m)

Enter valid values in all required fields.

What this formula means

Hydraulic diameter lets many circular-pipe correlations be applied to noncircular passages. It is four times flow area divided by wetted perimeter. For a rectangular duct it reduces to 2ab/(a+b).

Use hydraulic diameter for rectangular ducts, annuli, open channels, heat exchangers, and compact flow passages. The result should be interpreted with consistent units and with the assumptions of the underlying model in mind.

Worked example

A 0.4 m by 0.2 m full rectangular duct has Dh = 0.2667 m.

Applications

Use hydraulic diameter for rectangular ducts, annuli, open channels, heat exchangers, and compact flow passages. It is also useful as an independent check when comparing hand calculations, laboratory measurements, simulations, or preliminary engineering estimates.

Assumptions and common mistakes

Important: Wetted perimeter includes only boundaries in contact with the fluid. For an open channel, the free surface is not part of the wetted perimeter.

Use SI units unless the field label states otherwise. Keep significant figures appropriate to the accuracy of the inputs, and do not treat a simplified calculator as a replacement for codes, detailed design standards, or professional review.

Frequently asked questions

What equation does this calculator use?Dh = 4A / Pw. The calculator evaluates the displayed relationship using the values entered above.

Are the results unit-sensitive?Yes. The input labels use SI units so the displayed output follows the corresponding SI unit.

Can I use this for engineering design?Use it for learning and preliminary checks. Final designs require validated data, applicable standards, safety factors, and professional judgment.