Duct Velocity
Calculate velocities in circular and rectangular ducts  imperial and SIunits  online calculator.
Air Duct Velocity  Imperial Units
Air velocity in a ventilation duct can be calculated in imperial units as
v _{ i } = q _{ i } / A _{ i }
= q _{ i } / [ π (d _{ f } / 2) ^{ 2 } )]
= q _{ i } / [ π ((d _{ i } / 12) / 2) ^{ 2 } )]
= (576 / π) ( q _{ i } / d _{ i } ^{ 2 } )
= 144 q _{ i } / (a _{ i } b _{ i } ) (1)
where
v _{ i } = air velocity (ft/min)
q _{ i } = air flow (ft ^{ 3 } /min)
A _{ i } = area of duct (ft ^{ 2 } )
d _{ f } = diameter of duct (ft)
d _{ i } = diameter of duct (in)
a _{ i } = width of duct (in)
b _{ i } = 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
v _{ i } = (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 ).
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Air Duct Velocity  SI Units
Air velocity in a duct can be calculated in SI units
v _{ m } = q _{ m } / A _{ m }
= q _{ m } / (π (d _{ m } / 2) ^{ 2 } )
= 4 q _{ m } / (π d _{ m } ^{ 2 } )
= q _{ m } / (a _{ m } b _{ m } ) (2)
where
v _{ m } = air velocity (m/s)
q _{ m } = air flow (m ^{ 3 } /s)
A _{ m } = area of duct (m ^{ 2 } )
d _{ m } = diameter of duct (m)
a _{ m } = width of duct (m)
b _{ m } = 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 m ^{ 3 } /s can be calculated as
v _{ m } = (1 m ^{ 3 } /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 m ^{ 3 } /h) , duct size 8 in (200 mm) and velocity 1150 fpm (5.8 m/s) .
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