Duct CFM Calculator
How much air a duct can carry at a target velocity.
Enter your values
ⓘ Live results
Results
- Airflow capacity
- 279 CFM
- Free area
- 0.349 sq ft
- Equivalent round diameter
- 8 in
- In m³/h
- 474
Working
- 10.349 sq ft x 800 FPM = 279 CFM
Quick answer
Duct capacity is the free area multiplied by the design velocity you allow.
CFM = area (sq ft) x design velocity (FPM)
How to use the Duct CFM Calculator
- 1Pick the duct shape and enter its inside dimensions.
- 2Set a design velocity — 700–900 FPM for quiet residential branches.
- 3Compare the capacity with the CFM that branch actually needs.
Worked example
Using the values the calculator loads with:
Inputs
- Duct shapeRound
- Diameter (round)8 in
- Width (rect)12 in
- Height (rect)8 in
- Design velocity800 FPM
Results
- Airflow capacity279 CFM
- Free area0.349 sq ft
- Equivalent round diameter8 in
- In m³/h474
- 0.349 sq ft x 800 FPM = 279 CFM
Frequently asked questions
How much CFM does 6 inch duct carry?
About 110–140 CFM at a quiet 600–700 FPM, and up to 175 CFM if you accept more noise.
What is equivalent diameter for?
It lets you compare a rectangular duct against round duct friction charts.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.