Rectangular duct is sheet metal ductwork with a four-sided cross-section, fabricated from flat galvanized steel sheets joined at four longitudinal seams. While round spiral duct has become the preferred choice for new commercial construction, rectangular duct remains essential for low-clearance installations, renovation projects, and applications requiring internal duct liner for acoustic treatment.
This guide covers everything an HVAC engineer or contractor needs to specify, fabricate, or install rectangular ductwork — from sizing and gauge selection to reinforcement requirements and cost comparison with round alternatives.
Standard Rectangular Duct Sizes
Rectangular duct dimensions follow 2-inch increments for both width and height. The first number is always the wider dimension. Common sizes for commercial HVAC:
| Size (W×H) | Area (sq in) | Equiv. Round (in) | CFM @ 800 FPM | Aspect Ratio | Typical Use |
|---|---|---|---|---|---|
| 8×6 | 48 | 7 | 267 | 1.3:1 | Branch runs |
| 12×8 | 96 | 10 | 533 | 1.5:1 | Small trunk, soffits |
| 16×10 | 160 | 13 | 889 | 1.6:1 | Medium trunk |
| 20×12 | 240 | 16 | 1,333 | 1.7:1 | Main trunk |
| 24×14 | 336 | 19 | 1,867 | 1.7:1 | Main trunk |
| 30×16 | 480 | 23 | 2,667 | 1.9:1 | Large commercial |
| 36×18 | 648 | 27 | 3,600 | 2.0:1 | AHU connections |
| 48×24 | 1,152 | 36 | 6,400 | 2.0:1 | Major distribution |
Gauge Selection
Sheet metal gauge for rectangular duct depends on two factors: the longest side dimension and the pressure class. Heavier gauge prevents panel deflection (oil-canning) and structural failure under positive or negative pressure.
| Longest Side (in) | 2" w.g. (Low P) | 4" w.g. (Med P) | 6" w.g. (High P) |
|---|---|---|---|
| Up to 12 | 26 ga | 24 ga | 22 ga |
| 13-30 | 24 ga | 22 ga | 20 ga |
| 31-60 | 22 ga | 20 ga | 18 ga |
| 61-84 | 20 ga | 18 ga | 16 ga |
For panels wider than 18 inches in low-pressure systems, cross-breaking (pressing a diagonal crease into each panel) is required to stiffen the flat surface and prevent booming/oil-canning. Panels wider than 30 inches require external angle reinforcement at regular intervals per SMACNA tables.
Aspect Ratio: The Critical Design Rule
The aspect ratio is the ratio of width to height. A 24×12 duct has an aspect ratio of 2:1. SMACNA recommends keeping the aspect ratio at or below 4:1 for the following reasons:
Material waste. A 24×6 duct (4:1) uses 60 inches of perimeter for 144 sq in of area. A 16×9 duct (1.8:1) uses only 50 inches of perimeter for the same 144 sq in. That is 20% more metal for the same airflow capacity — pure waste.
Pressure drop. High-aspect-ratio ducts have higher friction loss per foot than equivalent round or low-aspect-ratio ducts because the hydraulic diameter is smaller. A 24×6 duct has the same friction loss as a 9.6-inch round duct, while a 16×9 has friction equivalent to a 11.5-inch round — significantly better.
Noise. Wide, flat panels vibrate more easily, producing low-frequency booming noise (rumble) that is difficult to attenuate. Aspect ratios above 3:1 in occupied areas almost always require external reinforcement to control panel resonance.
Rectangular vs Round Duct: When to Choose Each
Choose rectangular duct when:
Ceiling plenum height is limited — a 20×8 rectangular duct is only 8 inches tall but handles the airflow of a 13-inch round duct. Renovation projects where existing rectangular openings, furr-downs, or chases cannot accommodate round duct. Internal duct liner is needed for acoustic treatment — liner is easier to install in rectangular duct than round. The duct must connect directly to equipment with rectangular discharge (many AHUs have rectangular connections).
Choose round duct when:
New construction where ceiling clearance is not constrained. Energy efficiency matters — round duct has 5-15% lower pressure drop and 50-80% less air leakage than rectangular at the same sealing effort. Budget is a concern — spiral duct installed cost is typically 30-50% less than rectangular. Exposed ductwork is desired for architectural aesthetics.
Transitioning Between Round and Rectangular
Most HVAC systems use both duct shapes — rectangular for main trunks in tight plenums and round for branch runs. The transition fitting between the two must be designed carefully to minimize pressure loss:
Transition angle. The included angle of the transition should not exceed 30° on any side. A steeper angle creates flow separation, turbulence, and noise. For a 24×12 rectangular duct transitioning to a 16-inch round, the transition piece should be at least 12 inches long.
Tap connections. Round branch takeoffs from rectangular trunk lines use either conical taps (preferred — lower pressure drop) or straight spin-in collars (more economical). The branch connection should include a volume damper for balancing.
Flex duct connections. Where rectangular trunk lines feed flexible duct branch runs, use a sheet metal round collar on the rectangular trunk with a crimped end for flex duct attachment. Secure the flex duct with a strap clamp and seal with mastic.
Whether you need round flex duct for branch runs or accessories for your rectangular system, browse our complete product catalog or request a quote for custom requirements.