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Calculation Machine

Whole House Fan CFM Calculator

Fan size and attic vent area for whole-house cooling.

Reviewed by the Calculation Machine team · How we check our maths

Your values

ⓘ Live results

Results
Fan size
4,800 CFM
House volume
14,400 cu ft
Attic exhaust free area
6.4 sq ft (922 sq in)
Window opening area
12 sq ft total
CFM per sq ft
2.67

Quick answer

Whole house fans are sized for 15–23 air changes per hour, roughly 2–3 CFM per square foot of living space.

The formula

CFM = floor area x height x ACH / 60

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Living area1800 sq ft
  • Average ceiling height8 ft
  • Target air changes20 ACH

Result

  • Fan size4,800 CFM
  • House volume14,400 cu ft
  • Attic exhaust free area6.4 sq ft (922 sq in)
  • Window opening area12 sq ft total
  • CFM per sq ft2.67

How to use the Whole House Fan CFM Calculator

  1. 1Enter the conditioned living area and average ceiling height.
  2. 2Pick 15 ACH for mild climates or 20–23 for hot ones.
  3. 3Open enough windows and provide the attic exhaust area shown before running the fan.

Frequently asked questions

What happens with too little attic venting?

Pressure builds, airflow drops and the fan gets loud — you need roughly 1 sq ft of net free attic vent per 750 CFM.

When should I run it?

When outdoor air is cooler than indoor, usually evening through early morning.

About this calculator