Construction
Ventilation CFM Calculator
Estimate required ventilation airflow in cubic feet per minute from room size and air changes.
The result is the rough airflow the room needs in cubic feet per minute, so a higher number means a stronger fan or ventilation unit is needed.
Formula
Room volume = area multiplied by ceiling height. Required CFM = volume multiplied by air changes per hour, divided by 60 minutes.
How it works
Multiply floor area by ceiling height for volume, then apply the air changes per hour you need and divide by sixty to get cubic feet per minute. Use the result as a rough guide when comparing fans or ventilation units, and check local ventilation guidance for the room type.
Worked example
For example, a room of 200 square feet with a ceiling height of 9 feet has a volume of 200 × 9 = 1,800 cubic feet, and at 2 air changes per hour the required airflow is 1,800 × 2 ÷ 60 = 60 CFM.
FAQs
What ACH should I choose?
Bedrooms and living rooms need fewer changes than kitchens, bathrooms and workshops. Check guidance for your room type.
Is this the fan size to buy?
It is a rough airflow target. Duct losses and fan ratings mean the installed system should be checked by a professional.
Can I use metric units here?
This calculator uses feet and CFM. For metric results, use the air-changes calculator with cubic metres per hour.
Rough estimate only — consult a licensed professional for structural work before building.
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