# Duct Velocity

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

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The velocity in ventilation ducts can be calculated in imperial or SI units.

### Duct Velocity in Imperial Units

The velocity of air in a ventilation duct can be expressed in imperial units like

v_{i}= q_{i}/ A_{i}

= 576 q_{i}/ (π d_{i}^{2})

= 144 q_{i}/ (a_{i}b_{i}) (1)

where

v_{i}= air velocity (ft/min)

q_{i}= air flow (cfm)

A_{i}= area of duct (square feet)

d_{i}= diameter of duct (inches)

a_{i}= width of duct (inches)

b_{i}= width of duct (inches)

#### 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 (1000 cfm) / (π (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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### Duct Velocity in SI - Units

Air velocity in a duct can alternatively be expressed in SI units like

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*).

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