Whole House Fan Size Calculator
Estimate whole-house fan airflow from building volume and target air changes, then compare installed fan capacity and operating cost.
Default project preview
4,800 CFM
Required per fan
4,800 CFM
Installed capacity
5,000 CFM
Capacity margin
+200 CFM
One air change
2.88 min
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Conditioned floor area
Enter this project assumption in ft². The current imperial default is 1800 ft².
Average ceiling height
Enter this project assumption in ft. The current imperial default is 8 ft.
Target air changes
Enter this project assumption in ACH. The current imperial default is 20 ACH.
Fan count
Enter this project assumption in a unitless value. The current imperial default is 1. Minimum: 1.
Rated capacity per fan
Enter this project assumption in CFM. The current imperial default is 5000 CFM.
Power per fan
Enter this project assumption in W. The current imperial default is 600 W.
Operating time
Enter this project assumption in hours/day. The current imperial default is 4 hours/day. Minimum: 0. Maximum: 24.
Electricity price
Enter this project assumption in per kWh. The current imperial default is 0.18 per kWh. Minimum: 0.
How to read the outputs
Required per fan
1 fan. Default preview: 4,800 CFM.
Installed capacity
entered ratings. Default preview: 5,000 CFM.
Capacity margin
above target. Default preview: +200 CFM.
One air change
at installed capacity. Default preview: 2.88 min.
Daily electricity
4 operating hours. Default preview: 2.4 kWh.
Daily energy cost
entered electricity rate. Default preview: $0.43.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Building volume = floor area × average ceiling height. Required airflow = volume × target ACH ÷ 60 for CFM, or volume × ACH for m³/h.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: August 19, 2026
Estimate required whole-house-fan airflow from building volume and target air changes, compare installed fan capacity, and estimate operating energy/cost.
Calculation steps
- 1Multiply conditioned floor area by average ceiling height for building volume.
- 2Multiply volume by target ACH and divide by 60 to obtain required CFM.
- 3Compare the required airflow with total rated capacity of the selected fans.
- 4Use fan watts and runtime to estimate electricity use independently of airflow.
Worked numerical example
An 1,800 ft² home with 8 ft average ceilings contains 14,400 ft³. At a target 20 air changes per hour, required airflow is 4,800 CFM. One 5,000 CFM fan slightly exceeds that entered target.
Purchasing and rounding
Fan capacity must be coordinated with adequate open-window area, attic exhaust, noise goals, controls and safe combustion-appliance operation. Follow manufacturer installation requirements.
Interpret the result
The result compares an entered ACH target with rated fan airflow. It does not guarantee comfort, cooling effectiveness or safe building pressure under all operating conditions.
Page-specific assumptions
- Average ceiling height reasonably represents building volume.
- Rated fan capacities can be summed.
- Target ACH is selected for the intended operating strategy.
Common mistakes
- Running a large fan without sufficient makeup-air openings.
- Ignoring required attic exhaust area.
- Treating outdoor air temperature/humidity as irrelevant to operation.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Whole House Fan Size Calculator questions
Why does a whole-house fan need attic exhaust area?+
Supply air entering the attic must have a safe path outdoors to prevent excessive pressure.
Can the fan run with closed windows?+
Operating instructions usually require suitable open inlets; follow the manufacturer and installer guidance.
Does this replace HVAC cooling?+
No. It exchanges indoor air with outdoor air and works only when outdoor conditions are suitable.