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Calculation Machine

Duct Velocity Calculator

Air speed in a duct from CFM and duct size.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Air velocity
1,100 FPM
Duct area
0.545 sq ft
In m/s
5.59
Noise check
Typical trunk velocity

Working

  1. 1600 CFM / 0.545 sq ft = 1,100 FPM

Quick answer

Duct velocity equals airflow in CFM divided by the duct's free area in square feet.

The formula

FPM = CFM / area (sq ft)

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Airflow600 CFM
  • Duct shapeRound
  • Round diameter10 in
  • Rect width12 in
  • Rect height8 in

Result

  • Air velocity1,100 FPM
  • Duct area0.545 sq ft
  • In m/s5.59
  • Noise checkTypical trunk velocity

How to use the Duct Velocity Calculator

  1. 1Enter the airflow the duct carries in CFM.
  2. 2Choose the duct shape and enter its inside dimensions.
  3. 3Keep residential branches under about 900 FPM to stay quiet.

Frequently asked questions

What velocity is too high?

Above roughly 1,500 FPM in residential duct you will hear rush noise at the registers.

Does insulation change it?

Only if it is internal liner — use the inside clear dimensions in that case.

About this calculator