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Calculation Machine

CFM Calculator

Airflow in cubic feet per minute from room size and air changes.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Required airflow
224 CFM
Room volume
2,240 cu ft
In m³/h
381 m³/h
In L/s
106 L/s
Minutes per full air change
10

Working

  1. 1volume 2,240 cu ft x 6 ACH / 60 = 224 CFM

Quick answer

CFM is room volume in cubic feet multiplied by air changes per hour, divided by 60.

The formula

CFM = (length x width x height x ACH) / 60

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Room length20 ft
  • Room width14 ft
  • Ceiling height8 ft
  • Air changes per hour6 ACH

Result

  • Required airflow224 CFM
  • Room volume2,240 cu ft
  • In m³/h381 m³/h
  • In L/s106 L/s
  • Minutes per full air change10

How to use the CFM Calculator

  1. 1Measure the room length, width and ceiling height in feet.
  2. 2Pick the air changes per hour your space needs — 6 ACH suits a living space, 8–15 a kitchen or workshop.
  3. 3Size the fan or duct for at least the CFM shown, then add 10–20% for duct and filter losses.

Frequently asked questions

What is a good CFM for a room?

Most habitable rooms are comfortable at 4–6 air changes per hour, which for a 20 x 14 ft room with 8 ft ceilings is roughly 190–280 CFM.

Does duct length change the CFM I need?

It does not change the air the room needs, but long or restrictive duct runs cut the fan's delivered CFM, so oversize the fan by 10–25%.

About this calculator