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Calculation Machine

Attic Fan CFM Calculator

Powered attic ventilator size and intake area.

Reviewed by James Whitfield · How we check our maths

Your values

ⓘ Live results

Results
Fan size
1,300 CFM
Base requirement
980 CFM
Intake vent free area
622 sq in
Air changes per hour
14 ACH (4 ft average height)

Quick answer

Attic fans are sized at about 0.7 CFM per square foot of attic floor, with extra for dark or steep roofs.

The formula

CFM = attic area x 0.7 x roof factor

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Attic floor area1400 sq ft
  • Roof pitchMedium (4:12–8:12)
  • Roof colourDark

Result

  • Fan size1,300 CFM
  • Base requirement980 CFM
  • Intake vent free area622 sq in
  • Air changes per hour14 ACH (4 ft average height)

How to use the Attic Fan CFM Calculator

  1. 1Enter the attic floor area, usually the same as the top-floor footprint.
  2. 2Select the roof pitch and colour, which both raise the heat load.
  3. 3Provide at least the intake free area shown, or the fan will pull air from the house.

Frequently asked questions

Do attic fans save money?

They can lower attic temperature, but without adequate soffit intake they pull conditioned air out of the home and cost more than they save.

Passive or powered?

Ridge and soffit venting is usually enough; powered fans suit hot climates with poor passive airflow.

About this calculator