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Calculation Machine

FPM to CFM Calculator

Airflow from measured velocity and duct area.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Airflow
491 CFM
Duct area
0.545 sq ft
In m³/h
834
In L/s
231.7

Working

  1. 1900 FPM x 0.545 sq ft = 491 CFM

Quick answer

CFM equals velocity in feet per minute multiplied by duct area in square feet.

The formula

CFM = FPM x area (sq ft)

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Duct shapeRound
  • Measured velocity900 FPM
  • Diameter (round)10 in
  • Width (rect)14 in
  • Height (rect)8 in

Result

  • Airflow491 CFM
  • Duct area0.545 sq ft
  • In m³/h834
  • In L/s231.7

How to use the FPM to CFM Calculator

  1. 1Take an average velocity reading across the duct, not a single centre point.
  2. 2Enter the duct's internal dimensions.
  3. 3Read the airflow, then compare it with the design CFM for that branch.

Frequently asked questions

How do I average the reading?

Use a traverse: take readings on a grid of equal-area points and average them.

Is centre velocity accurate?

Centre velocity runs high; multiply by about 0.9 for a rough duct average if you cannot traverse.

About this calculator