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HVAC & Airflow

CFM to FPM Calculator

Air velocity in feet per minute from CFM and duct size.

Enter your values

ⓘ Live results

Results
Air velocity
1,146 FPM
Duct free area
0.349 sq ft
In m/s
5.82
In mph
13.02
Noise band
typical commercial

Working

  1. 1400 CFM / 0.349 sq ft = 1,146 FPM

Quick answer

Velocity in feet per minute is CFM divided by the duct's free area in square feet.

FPM = CFM / area (sq ft)

How to use the CFM to FPM Calculator

  1. 1Choose round or rectangular duct.
  2. 2Enter the airflow and the duct dimensions in inches.
  3. 3Keep residential branches under about 900 FPM to stay quiet.

Worked example

Using the values the calculator loads with:

Inputs

  • Duct shapeRound
  • Airflow400 CFM
  • Diameter (round)8 in
  • Width (rect)12 in
  • Height (rect)8 in

Results

  • Air velocity1,146 FPM
  • Duct free area0.349 sq ft
  • In m/s5.82
  • In mph13.02
  • Noise bandtypical commercial
  1. 400 CFM / 0.349 sq ft = 1,146 FPM

Frequently asked questions

What velocity is too high?

Above roughly 1500 FPM residential duct becomes audible and static pressure climbs quickly.

Does insulation reduce free area?

Yes — for internally lined duct, measure the inside clear dimension, not the outer sheet metal size.

About this calculator

Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.