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Duct Velocity

Calculate velocities in circular and rectangular ducts - imperial and SI-units - online calculator

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air duct

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)


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).

Air volume - qi - (cfm)

Diameter - di - (inches)

or alternatively

Length side - ai - (inches)

Length side - bi - (inches)

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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)


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 volume - qm - (m3/s)

Diameter - dm - (m)

or alternatively

Length side - am - (m)

Length side - bm - (m)

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).

Air Ducts - Velocity Diagram

Download and print Air Ducts - Velocity Diagram!

Air Ducts Friction Loss

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