Rectangular to Round Duct Converter
Converts rectangular and flat oval duct to circular equivalent diameter using the Huebscher equation from ASHRAE Fundamentals Chapter 21, and works in reverse — give it a round duct and a ceiling void depth, and it lists every rectangular size that carries the same air at the same friction rate, with the aspect ratio of each.
Circular equivalent diameter
18.28 in
Nearest standard round 18.0 in · aspect ratio 2.00:1
| Cross-sectional area | 0.1858 m² · 2.000 ft² |
| Perimeter | 72.0 in |
| Hydraulic diameter 4A/P | 16.0 in |
| Velocity in the real section | 5.08 m/s · 1,000 fpm |
| Velocity in the equivalent round duct | 5.57 m/s · 1,097 fpm |
Equivalent diameter is an equal-friction, equal-airflow basis, so the equivalent round duct has less area than the section it represents and its velocity is higher. Use the real section velocity for noise and terminal selection.
The trap in the words "equivalent diameter"
Equivalent diameter sounds like it should mean "the round duct with the same area". It does not, and reading it that way produces errors that survive all the way to site. A 600 × 300 duct has an area of 0.18 m². Its circular equivalent is 457 mm, and a 457 mm round duct has an area of 0.164 m² — about 9 per cent less. The two ducts are equivalent in the only sense the equation claims: carrying the same air, they lose the same pressure per metre.
The practical consequence shows up whenever someone quotes a velocity. Push 950 L/s through that rectangular duct and the air moves at 5.3 m/s. Push it through the 457 mm equivalent and it moves at 5.8 m/s. Both numbers are correct; they answer different questions. Friction calculations want the equivalent. Noise, diffuser selection and anything an acoustician will read want the real section velocity. This tool prints both, side by side, precisely so the wrong one does not get copied onto a drawing.
How to use the converter
- Pick the direction. Rectangular to round for a duct you already have, round to rectangular when a sized round duct has to fit a shallow void, or flat oval to round for the third shape.
- Enter the internal dimensions. Internal clear dimensions, not the outside of the insulation and not the nominal drawing size. The equation works on the air path.
- Add the airflow if you want velocity. Optional. Entering it returns the velocity in the real section and in the equivalent round duct side by side, which is where most of the confusion about equivalent diameter lives.
- Set the void depth when converting to rectangular. The maximum height available filters the options table so only sizes that physically fit are listed.
- Read the equivalent diameter and the aspect ratio together. A size that matches on equivalent diameter but sits at 7:1 is not a size you want to build. The aspect ratio column is there to stop that.
The equations
- Huebscher (rectangular): De = 1.30 × (a·b)0.625 ÷ (a+b)0.250
- Flat oval: De = 1.55 × A0.625 ÷ P0.250
- Flat oval area: A = πa²/4 + a(W − a), with a the minor axis and W the major
- Flat oval perimeter: P = πa + 2(W − a)
- Hydraulic diameter: Dh = 4A ÷ P — reported for reference, not used for friction
Worked example
A 400 × 200 mm duct. The product a·b = 80,000 mm² and the sum a+b = 600 mm. Then De = 1.30 × 80,0000.625 ÷ 6000.250 = 1.30 × 1159.9 ÷ 4.9492 = 304.7 mm. Reverse the sides to 200 × 400 and the product and sum are unchanged, so the answer is identical. In imperial, a 24 × 12 in duct gives 1.30 × 2880.625 ÷ 360.250 = 18.28 in.
Published equivalent-diameter tables often round to the nearest whole number, so a printed table may show 302 or 305 mm for the same duct. The difference is table rounding, not a different equation.
Aspect ratio is where the money goes
A 1200 × 200 duct and a 500 × 480 duct enclose almost the same area, but the flat one has 2800 mm of perimeter against 1960 mm. That extra perimeter is extra sheet metal, extra insulation, extra reinforcement and extra hanger work on every metre of the run — and it carries a friction penalty on top, because friction scales with wetted perimeter for a given area.
- At or below 4:1 — normal territory. Cost and friction penalties are modest.
- 4:1 to 6:1 — acceptable when the void genuinely forces it, but price it properly and expect the fabricator to add reinforcement.
- Above 6:1 — treat as a hard limit. Two parallel ducts at a sane ratio are usually cheaper to build, easier to hang and quieter to run than one very flat one.
Frequently asked questions
What is the circular equivalent diameter of a duct?
It is the round duct that loses the same pressure per metre as your rectangular duct while carrying the same airflow. It is defined on an equal-friction, equal-airflow basis, which is not the same as equal area — and that distinction is the source of most of the confusion around it. The relationship used here is the Huebscher equation from ASHRAE Fundamentals Chapter 21.
Why does the equivalent round duct have a smaller area than my rectangular duct?
Because a rectangular section is a less efficient shape for moving air. It has more wetted perimeter for the area it encloses, so it generates more friction, and to match that friction the equivalent round duct must be smaller and therefore faster. This is exactly why round duct is cheaper to run: for the same air and the same pressure loss it needs less metal.
Which velocity should I use for noise calculations?
The velocity in the real section — airflow divided by the actual rectangular or flat oval area. The equivalent round velocity is an artefact of the friction calculation and is always higher. Quoting it to an acoustician, or using it to select a diffuser, will give you numbers that do not match what the duct actually does.
Does 600 x 300 give the same answer as 300 x 600?
Yes, exactly the same. The Huebscher equation sees only the product of the two sides and their sum, so orientation makes no difference to the friction. Orientation matters enormously for whether the duct fits the ceiling void and how it is supported, but not for the air.
Why does the rectangular duct always need more area than the round one it replaces?
Same reason as above, seen from the other side. When you convert a round duct to a rectangular equivalent, the rectangular size has to enclose more area to deliver the same friction performance — typically 10 to 20 per cent more at sensible aspect ratios, and much more as the section gets flatter. Check that extra width fits before committing to it on a drawing.
When is flat oval worth the money?
When the void is too shallow for round but you want most of round duct performance. Flat oval keeps a far better perimeter-to-area ratio than a flat rectangular duct of the same depth, so it carries less friction penalty. It costs more to fabricate than either round or rectangular and needs specialist fittings, so it earns its place only where depth is genuinely the binding constraint.
Last updated: 26 July 2026