# 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

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*).

Air volume- q_{i}- (cfm)

Diameter- d_{i}- (inches)or alternatively

Length side- a_{i}- (inches)

Length side -b_{i}- (inches)

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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 volume- q_{m}- (m^{3}/s)

Diameter- d_{m}- (m)or alternatively

Length side- a_{m}- (m)

Length side -b_{m}- (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 m ^{3}/h)*, duct size

*8 in (200 mm)*and velocity

*1150 fpm (5.8 m/s)*.

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