Rectangular Duct for Round Equivalent
Width of rectangular duct (at your height limit) matching a round duct's friction.
A flat duct needs MORE area than the round it replaces: a 400 mm round in a 250 mm ceiling space becomes ~560×250. Aspect ratio is money.
Formula
Rectangular Duct for Round Equivalent is a free rectangular equivalent duct for HVAC designers, technicians and energy auditors — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Rectangular Duct for Round Equivalent
Width of rectangular duct (at your height limit) matching a round duct's friction. The calculation implements D_e = 1.30·(ab)^0.625/(a+b)^0.25 (Huebscher) — solved for width (ASHRAE Fundamentals, ch. 21; SMACNA). A flat duct needs MORE area than the round it replaces: a 400 mm round in a 250 mm ceiling space becomes ~560×250. Aspect ratio is money.
How to use Rectangular Duct for Round Equivalent
- 1Enter Round duct to match in mm.
- 2Enter Available height in mm.
- 3Read Required width, Aspect ratio instantly — no submit button needed.
- 4Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Rectangular Duct for Round Equivalent?
- ✓Implements the standard formula — D_e = 1.30·(ab)^0.625/(a+b)^0.25 (Huebscher) — solved for width
- ✓Reference cited on-page: ASHRAE Fundamentals, ch. 21; SMACNA
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Built-in engineering verdict flags out-of-range results instantly
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Rectangular Duct for Round Equivalent use?+
It computes D_e = 1.30·(ab)^0.625/(a+b)^0.25 (Huebscher) — solved for width, per ASHRAE Fundamentals, ch. 21; SMACNA. The formula is displayed under the result.
What should I keep in mind when using this calculator?+
A flat duct needs MORE area than the round it replaces: a 400 mm round in a 250 mm ceiling space becomes ~560×250. Aspect ratio is money.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Rectangular Duct for Round Equivalent free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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