SettingsSettings for this calculationUS
One side length of the rectangular duct cross-section.
Measure the actual sheet metal-to-sheet metal internal dimension, not the nominal or outside dimension. Side A and Side B are interchangeable — the Huebscher equation is symmetric, so it doesn't matter which side you enter first.
The other side length of the rectangular duct cross-section.
Measure the actual sheet metal-to-sheet metal internal dimension, not the nominal or outside dimension. This equation is most reliable for aspect ratios (longer side ÷ shorter side) up to about 8:1 — very flat, extreme-aspect-ratio ducts fall outside where the equation was validated.
Equivalent round duct diameter
10.66 in
They open the calculator with your figures already in it
Rectangular-to-Round Equivalent Duct Diameter Calculator: 10.66 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Show calculation logicHide calculation logic
How this was calculated
Formula source(s)
- 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.
Inputs used
- Rectangular Duct Side A
- 12 in
- Rectangular Duct Side B
- 8 in
What this calculation does not cover
- The diameter that comes back is a clear internal bore. Internal acoustic lining takes it from both sides, so 25 mm (1 in) of liner means ordering roughly 50 mm (2 in) larger outside to keep the airflow, and flexible duct and duct board are rougher than the smooth galvanized steel the equation was fitted to — a flex run has to be larger again.
- Converts a size without checking it. If the rectangular duct was already undersized and running fast and noisy, the equivalent round comes back equally undersized: the equation matches friction loss between two shapes, it never says the friction loss was acceptable in the first place.
- Does not ask whether the round duct will physically go in. Ducts are rectangular because the space is shallow, and the equivalent diameter always exceeds the short side it replaces — a 300 x 150 duct works out near 230 mm (9 in) round, which no longer fits the ceiling void that forced the rectangle.
Add the equipment this sizes
This result is a specification — 10.66 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- 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.
Cite this page
Your workspace
Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.