Duct Size Calculator

Inputs

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Volumetric airflow through the duct.
m³/h
Design velocity used for velocity sizing.
m/s
Rectangular width divided by height. 2 means a duct twice as wide as it is high.
Straight duct length used to convert friction rate into pressure drop.
m
Equivalent absolute roughness. Galvanized steel duct is often screened near 0.09 mm.
mm
Air density at the duct condition. Room-temperature sea-level air is about 1.2 kg/m³.
kg/m³

Results

Default result
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Round duct diameter(D)
252.3mm
Pass

area sized to 5.00 m/s

Also computed

Rectangular width(W)316.2mm

Rectangular height(H)158.1mm

Rect. equivalent round diameter(D_eq)240.9mm

Round duct velocity(v_r)Pass5m/s

typical comfort-duct screening range

Rectangular duct velocity(v_rect)Pass5m/s

typical comfort-duct screening range

Round friction rate(Δp/L)Caution1.207Pa/m

moderate friction; check fan static pressure

Friction rate vs Round duct diameterRound-duct friction rate from Darcy-Weisbach. Smaller ducts raise both velocity and static-pressure loss quickly.Friction rate vs Round duct diameter01020300100200300400500selectedRound duct diameter (mm)Friction rate (Pa/m)
Round-duct friction rate from Darcy-Weisbach. Smaller ducts raise both velocity and static-pressure loss quickly.
Method notes 3 notes
  • velocity sizing: round and rectangular options are calculated from the same airflow and roughness assumptions.
  • Pressure loss uses Darcy-Weisbach with hydraulic diameter, room-temperature air viscosity and a Swamee-Jain turbulent friction factor.
  • Final HVAC duct design should also check fittings, transitions, diffusers, balancing dampers, acoustics, leakage, insulation, code requirements and fan static pressure.

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