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Rectangular-to-Round Equivalent Duct Diameter Calculator

Calculate the round duct diameter that delivers equivalent airflow capacity to a given rectangular duct cross-section, using the Huebscher equation.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-26 · v1.0.0

Market
Imperial · sales tax

Equivalent round duct diameter

10.66 in

High confidence

At the values currently entered, the equivalent round duct diameter works out to 10.7 in. Figures are shown for United States, where IRC 2024 is the governing residential reference; switch the market above if you are building elsewhere.

Add the equipment this sizes

This result is a specification — 10.656 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Huebscher equivalent diameter equation: De = 1.30 × (a×b)^0.625 ÷ (a+b)^0.25, where a and b are the rectangular duct's side lengths (inches) and De is the equivalent round duct diameter (inches) — the standard ASHRAE-cited method for converting between rectangular and round duct sizes at equal airflow and friction loss.

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Frequently asked questions

What is the Huebscher equation used for?
It converts a rectangular duct's cross-sectional dimensions into the diameter of a round duct that delivers equivalent airflow capacity and friction loss — the standard ASHRAE-cited method for comparing or substituting between rectangular and round duct sizes.
Does the equivalent diameter equal the rectangular duct's average side length?
No — the Huebscher equation accounts for the different way air friction scales with duct shape, so the equivalent round diameter is generally smaller than a simple average of the two rectangular sides.
Can I use this to size a round duct replacing an existing rectangular duct?
Yes — it gives the round duct diameter that matches the rectangular duct's airflow capacity and friction loss at the same air velocity, which is the standard approach when substituting duct shapes in a system.