Round vs Rectangular Ductwork: Selection, Conversion and Friction Loss
What You Need to Know
Round duct moves air more efficiently than rectangular duct of the same cross-sectional area. It has less friction loss per metre, less air leakage, and is cheaper to fabricate and seal. Rectangular duct exists for one reason: it fits in shallow ceiling spaces where round duct will not.
For the same airflow, a round duct has the smallest perimeter of any shape. A square duct (1:1 aspect ratio) has about 13% more perimeter than a round duct of equal area. A 4:1 rectangular duct has roughly 40% more perimeter. More perimeter means more wetted surface area, more friction, more sheet metal, and more sealed joints that can leak.
The conversion between the two shapes uses the Huebscher equation, which gives the round diameter that produces the same friction loss per metre at the same airflow. This is the basis for every "round to rectangular" duct calculator on the market, including the calculator on this site.
The rest of this guide covers when to use each shape, how the maths works, what aspect ratio limits to apply, and a reference table of common round sizes with their rectangular equivalents at 1:1, 2:1, and 3:1 aspect ratios.
Standards and Construction Requirements
- AS 4254.1, flexible duct covers flexible ductwork construction, materials, and installation. Flexible duct is round only and is intended for short final connections to diffusers, typically 1.5 metres maximum length. Long runs of flexible duct produce two to four times the friction loss of equivalent rigid round duct. (AS 4254.1:2012)
- AS 4254.2, rigid duct sets sheet metal thickness, joint construction, stiffening, and reinforcement requirements for rigid sheet metal ductwork in both round and rectangular form. Sheet thickness, stiffener spacing, and reinforcement class step up as the rectangular long side increases, which is why large rectangular ducts cost more per square metre of surface area than smaller ones. (AS 4254.2:2012)
- Air leakage classes in AS 4254.2 are A, B, C, and D, with D being the tightest. Spiral round duct typically achieves Class C or D with standard slip joints and gaskets. Rectangular duct typically achieves Class A or B unless every transverse and longitudinal seam is sealed with mastic and the joint is pressure tested. Tighter classes mean less fan power for the same delivered airflow. (AS 4254.2:2012, Section 5)
- AS 1668.2:2024, mechanical ventilation sets the airflow rates that the ductwork must carry. The shape of the duct does not change the required airflow, but it does change the static pressure the fan must overcome and the noise generated by the fan. (AS 1668.2:2024)
- NCC 2025, Section J energy efficiency requires duct insulation to specified R-values for conditioned air. Round duct has less surface area per metre than rectangular, so it has lower heat gain or loss for the same airflow. This reduces both insulation cost and energy waste. (NCC 2025, Section J)
- ASHRAE Handbook, Fundamentals (Duct Design chapter) is the international primary reference for duct sizing methods, friction charts, and the Huebscher equivalent diameter equation. Australian practice follows the ASHRAE methods, with airflow rates and ventilation requirements set by AS 1668.2. (ASHRAE Handbook, Fundamentals, Chapter on Duct Design)
- Industry aspect ratio limit for rectangular duct is 4:1 maximum, with 2:1 or lower preferred. Above 4:1, the friction penalty, sheet metal cost, and acoustic breakout become unacceptable. CIBSE Guide B and ASHRAE Fundamentals both recommend the same upper limit. (CIBSE Guide B3, ASHRAE Fundamentals)
The Maths: Hydraulic Diameter and Huebscher Conversion
Hydraulic diameter. Friction in any duct depends on the ratio of cross-sectional area to wetted perimeter. The hydraulic diameter (Dh) reduces any shape to an equivalent diameter for friction calculations.
For a round duct, Dh equals the actual diameter. For a rectangular duct of sides a and b, Dh equals 2ab divided by (a + b). Hydraulic diameter is useful for Reynolds number and rough friction estimates, but it is not what the industry uses for round-to-rectangular sizing. For sizing, use the Huebscher equation.
Huebscher equivalent diameter. The Huebscher equation gives the round duct diameter that produces the same friction loss per metre at the same airflow as a given rectangular duct. This is the standard equivalence used in every duct sizing tool, including the ASHRAE and CIBSE friction charts.
The equation works in either direction. To find the rectangular size for a given round diameter, fix one side (or fix the aspect ratio) and solve for the other. Most calculators iterate because the equation cannot be rearranged into a clean closed form for a given aspect ratio.
Worked example: convert 400 mm round to rectangular
Target a 2:1 aspect ratio. Set b = a/2. Substitute into the Huebscher equation and iterate.
- Try a = 500, b = 250: De = 1.30 × (500 × 250)0.625 / (750)0.250 = approximately 393 mm. Slightly small.
- Try a = 510, b = 255: De ≈ 400 mm. Match.
Round to a standard manufacturing size: 500 by 250 mm rectangular is the practical equivalent of 400 mm round at a 2:1 aspect ratio. The two ducts carry the same airflow at the same friction loss per metre.
Worked example: pressure drop comparison
Carry 1.0 m³/s of air at 6 m/s mean velocity through a 30 metre run.
- Round option: 460 mm diameter spiral, friction loss approximately 0.85 Pa/m, total = 25.5 Pa.
- Rectangular option (2:1): 600 by 300 mm, friction loss approximately 1.0 Pa/m, total = 30 Pa.
- Rectangular option (4:1): 1000 by 250 mm, friction loss approximately 1.3 Pa/m, total = 39 Pa.
Same airflow, same velocity, very different fan static pressure. The 4:1 rectangular run needs about 50% more fan static pressure than the round run, plus more sheet metal, more sealing labour, and noticeably more breakout noise.
Reference Table: Round to Rectangular Equivalents
Each rectangular dimension below produces approximately the same friction loss per metre at the same airflow as the round size in the first column, calculated from the Huebscher equation. Round to the nearest standard manufacturing size when ordering.
| Round (mm) | Rect. 1:1 (mm) | Rect. 2:1 (mm) | Rect. 3:1 (mm) |
|---|---|---|---|
| 200 | 180 × 180 | 250 × 125 | 335 × 110 |
| 250 | 225 × 225 | 320 × 160 | 425 × 140 |
| 315 | 285 × 285 | 400 × 200 | 540 × 180 |
| 400 | 365 × 365 | 510 × 255 | 685 × 230 |
| 500 | 455 × 455 | 640 × 320 | 855 × 285 |
| 630 | 575 × 575 | 805 × 405 | 1080 × 360 |
| 800 | 730 × 730 | 1025 × 515 | 1370 × 460 |
| 1000 | 915 × 915 | 1280 × 640 | 1715 × 575 |
Values rounded to the nearest 5 mm. For exact figures or non-standard aspect ratios, use the CCC duct calculator.
Key Design Decisions
Round Spiral vs Rectangular as the Default Trunk
Specify spiral round duct as the default for trunks and risers wherever the ceiling depth allows. Spiral round duct has lower friction, lower leakage, lower noise, and lower installed cost per delivered cubic metre per second. Rectangular trunks should only appear where ceiling depth, plenum returns, or coordination with structure forces a flat profile.
Aspect Ratio Limit
Hold the rectangular aspect ratio at or below 2:1 wherever possible. Accept up to 3:1 in tight ceiling zones. Treat 4:1 as the absolute maximum and only where the architect has no other option. Above 4:1, the friction penalty, sheet metal usage, and acoustic breakout all rise sharply and the duct becomes harder to support and seal.
Flexible vs Rigid for Final Connections
Use flexible duct only for the last short connection to the diffuser, no more than 1.5 metres uncompressed length, and pulled fully taut. Long flexible duct runs (3 metres or more) commonly produce two to four times the friction of equivalent rigid round duct, especially when sagged or compressed during installation. This is one of the most common causes of "the system is balanced on paper but the room is short on air" complaints.
Sealing and Leakage Class
Specify a leakage class (A through D under AS 4254.2) and pressure test the system to verify it. Spiral round duct typically achieves Class C or D with little extra effort. Rectangular duct usually needs every transverse joint, longitudinal seam, and branch connection sealed with mastic to reach Class C. Unsealed rectangular ductwork can lose 15 to 30 percent of its airflow before it reaches the diffuser, which means the fan is sized too small or the rooms run short on air.
Who Needs to Know What
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References
- AS 4254.1:2012, Ductwork for Air-Handling Systems in Buildings, Part 1: Flexible Duct
- AS 4254.2:2012, Ductwork for Air-Handling Systems in Buildings, Part 2: Rigid Duct
- AS 1668.2:2024, The Use of Ventilation and Airconditioning in Buildings, Part 2: Mechanical Ventilation in Buildings
- National Construction Code 2025, Building Code of Australia, Volume One, Section J Energy Efficiency
- ASHRAE Handbook, Fundamentals, Chapter on Duct Design, latest edition
- CIBSE Guide B3, Heating, Ventilating, Air Conditioning and Refrigeration: Air Distribution Systems
- Huebscher, R.G., Friction Equivalents for Round, Square and Rectangular Ducts, ASHVE Transactions Vol. 54