Duct friction comparison

Equivalent Duct Diameter Calculator

Estimate an equal-friction round-diameter equivalent for a rectangular duct.

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Calculate equivalent round diameter

Enter inside rectangular dimensions. The output is an equal-friction approximation, not an equal-area substitute.

Equivalent round diameterinches

How to use

  1. Enter the inside width and height of the rectangular duct.
  2. Use the result as a preliminary round-equivalent comparison.
  3. Check the actual selected geometry, velocity, roughness, fittings, and full pressure path separately.

Formula

De = 1.30 × (W × H)0.625 ÷ (W + H)0.25

W and H are the rectangular inside dimensions in inches; De is the equivalent round diameter in inches. This is commonly called the Huebscher equal-friction approximation.

Example

For a 24 in × 12 in rectangular duct, the approximation gives about 18.1 in equivalent round diameter. The 24 × 12 rectangle does not have the same face area or velocity as an 18.1 in round duct at a given CFM.

Result interpretation

The calculation is useful for preliminary friction comparison, not for replacing a full duct method. Lining, roughness, fittings, transitions, takeoffs, dampers, terminals, density, airflow, fan pressure, and noise can all change the practical result.

Use the equivalent value carefully

Use the Rectangular Duct Size Calculator for face-area geometry and the Duct Friction Loss Calculator for a transparent straight-run check with stated inputs.

Frequently asked questions

Is equivalent diameter the same as hydraulic diameter?

No. Hydraulic diameter is a geometric measure. This equivalent diameter is an equal-friction approximation for comparing rectangular and round ducts.

Can I substitute the result directly for a round duct?

Not without checking actual available sizes, velocity, resistance, fittings, fan pressure, sound, and the complete system design.

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