# Duct Velocity

### 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 * ).

or alternatively

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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 * ).

or alternatively

### 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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## Related Topics

### • Ventilation Systems

Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.

## Related Documents

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Typical air duct velocities in applications like ventilation systems or compressed air systems.

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A major friction loss diagram for air ducts - Imperial units ranging *10 - 100 000 cfm*.

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The velocity reduction method can be used when sizing air ducts.

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### Sizing Circular Ducts

A rough guide to maximum air volume capacity of circular ducts in comfort, industrial and high speed ventilation systems.

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Standard dimensions of spiral ducts - Imperial units.

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Recommended air velocities in ventilation ducts

### Velocity Units Converter

Convert between speed and velocity units like *m/s, km/h, knots, mph and ft/s*.