# Duct Velocity

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

### Duct Velocity - Imperial Units

Air velocity in a ventilation duct can be calculated in imperial units as

vi = qi / Ai

= 576 qi / (π di2)

= 144 qi / (ai bi)                                   (1)

where

vi = air velocity (ft/min)

qi = air flow (cfm)

Ai = area of duct (square feet)

di = diameter of duct (inches)

ai = width of duct (inches)

bi = 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

vi = 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).

Air volume - qi - (cfm)

Diameter - di - (inches)

or alternatively

Length side - ai - (inches)

Length side - bi - (inches)

### Related Mobile Apps from The Engineering ToolBox

- free apps for offline use on mobile devices

### 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)

where

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)

## Related Topics

• Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more

## Search the Engineering ToolBox

- "the most efficient way to navigate!"

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!