Duct Velocity
Calculate velocities in circular and rectangular ducts - imperial and SI-units - online calculator.
Air Duct Velocity - Imperial Units
Air velocity in a ventilation duct can be calculated in imperial units as
vi = qi / Ai
= qi / [π (df / 2)2)]
= qi / [π ((di / 12) / 2)2)]
= (576 / π) (qi / di2)
= 144 qi / (ai bi) (1)
where
vi = air velocity (ft/min)
qi = air flow (ft3/min)
Ai = area of duct (ft2)
df = diameter of duct (ft)
di = diameter of duct (in)
ai = width of duct (in)
bi = width of duct (in)
Example - Air Flow Velocity in a Duct
The velocity in a 12 inch duct with air flow 1000 cfm can be calculated as
vi = (576 / π) (1000 cfm) / (12 in)2)
= 1273 (ft/min)
Air Flow Velocity Calculator - Imperial Units
Air velocity can be calculated with the calculator below. Add air volume - q - and diameter - d - (or length a and b).
Related Mobile Apps from The Engineering ToolBox 
- free apps for offline use on mobile devices
Air Duct Velocity - SI Units
Air velocity in a duct can be calculated in SI units
vm = qm / Am
= qm / (π (dm / 2)2)
= 4 qm / (π dm2)
= qm / (am bm) (2)
where
vm = air velocity (m/s)
qm = air flow (m3/s)
Am = area of duct (m2)
dm = diameter of duct (m)
am = width of duct (m)
bm = width of duct (m)
Example - Air Flow Velocity in a Duct
The velocity in a rectangular 0.5 m x 0.5 m duct with air flow 1 m3/s can be calculated as
vm = (1 m3/s) / ((0.5 m) (0.5 m))
= 4 (m/s)
Air Flow Velocity Calculator - SI Units
Air velocity can be calculated with the calculator below. Add air volume - q - and diameter - d - (or length a and b).
Air Ducts - Velocity Diagram
The chart below can be used to estimate velocities in air ducts.
The default values are for air flow 400 cfm (680 m3/h), duct size 8 in (200 mm) and velocity 1150 fpm (5.8 m/s).
Download and print Air Ducts - Velocity Diagram!
- make 3D HVAC models with the Engineering ToolBox Sketchup Extension
