HVAC & Airflow

CFM to FPM Calculator

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

Enter your values

Duct shape

Choose one of: Round, Rectangular.

Airflow(CFM)

Enter the airflow in CFM.

Diameter (round)(in)

Enter the diameter (round) in in.

Width (rect)(in)

Enter the width (rect) in in.

Height (rect)(in)

Enter the height (rect) in in.

Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.

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

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)

At a glance

What it does	Air velocity in feet per minute from CFM and duct size.
Category	HVAC & Airflow
Inputs needed	Duct shape, Airflow, Diameter (round), Width (rect), Height (rect)
Main output	Air velocity
Formula	FPM = CFM / area (sq ft)
Cost	Free — no sign-up, no download

How to use the CFM to FPM Calculator

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

Inputs explained

Duct shape: Choose one of: Round, Rectangular.
Airflow(CFM): Enter the airflow in CFM.
Diameter (round)(in): Enter the diameter (round) in in.
Width (rect)(in): Enter the width (rect) in in.
Height (rect)(in): Enter the height (rect) in in.

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

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.

What do I need to enter into the CFM to FPM Calculator?

Just 5 values: duct shape, airflow, diameter (round), width (rect), height (rect). Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.

How does the CFM to FPM Calculator work out the answer?

Velocity in feet per minute is CFM divided by the duct's free area in square feet. It applies the formula FPM = CFM / area (sq ft) and shows the working so you can check each step by hand.

Is the CFM to FPM Calculator free, and do I need an account?

It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.

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