Same cross-sectional area. Different performance. The shape matters.
The perimeter problem
Shape Area Perimeter vs Round
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Round (16.6") 216 in² 52.2 in baseline
Square (14.7²) 216 in² 58.8 in +13%
Rect (24×9) 216 in² 66.0 in +26%
Rect (36×6) 216 in² 84.0 in +61%
More perimeter = more friction, more noise, more leakage.
Equivalent diameter
Rectangular ducts have higher friction than round for the same area. Equivalent diameter tells you what round duct the rectangle behaves like:
De = 1.3 × (W × H)^0.625 / (W + H)^0.25
For 216 in² area:
Round 16.6": De = 16.6 in (actual diameter)
Rect 24×9: De = 15.6 in (acts 6% smaller)
Rect 36×6: De = 13.8 in (acts 17% smaller)
The 36×6 duct has the same area as the round but behaves like a duct 2.8 inches smaller.
When to use round
✓ Ceiling space allows it
✓ Noise-sensitive spaces (studios, hospitals, offices)
✓ Long straight runs (friction compounds over distance)
✓ Exposed ductwork (aesthetics)
✓ Spiral duct available (lower cost per foot)
When to use rectangular
✓ Ceiling height limited (9" plenum vs 16" round)
✓ Routing around beams and pipes
✓ Aspect ratio stays ≤ 4:1
✓ Multiple branches in tight spaces
The rule
Aspect ratio ≤ 4:1 → acceptable trade-off
Aspect ratio > 4:1 → friction penalty exceeds space savings
Aspect ratio > 6:1 → avoid (noise, fabrication cost, energy waste)
Noise comparison
Round duct: curved surface resists vibration
minimal panel drumming
preferred for NC 25-35 spaces
Rectangular duct: flat panels vibrate (oil-canning)
low-frequency rumble
needs heavier gauge or external bracing
for noise-sensitive applications
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