Garage Ventilation Calculator: Exhaust Fan CFM
Work out the exhaust airflow in CFM your garage needs from its volume and the air changes per hour you want.
A garage traps heat, humidity and fumes from paint, coatings and solvents. The calculator works out how much exhaust airflow replaces the air at the rate you choose, and how many fans of a given rating that takes.
When this tool helps most
Use it when choosing an exhaust fan, planning a workshop or finishing area, or trying to cool a hot garage in summer.
How to use this tool
- Enter the garage length, width and ceiling height.
- Choose how many air changes per hour you want.
- Enter the fan’s CFM rating at the pressure it will work against.
- Read the airflow needed and the number of fans.
Worked example
A 24 by 24 by 9 foot garage holds 5,184 cubic feet. At 6 air changes per hour it needs about 520 CFM, which is 2 fans rated at 400 CFM. Together they replace the air about every 6.5 minutes.
Understanding your results
Size the fan to the CFM figure and give it makeup air, such as a louver on the opposite wall. Higher air-change rates suit workshops, finishing and hot garages.
Common mistakes to avoid
- Installing an exhaust fan with no makeup air, which starves it.
- Using a fan’s free-air rating when it will push through ducts and grilles.
- Pulling air from the house through the connecting door of an attached garage.
- Relying on ventilation to run engines or generators indoors; never do that.
How to get the most accurate result
- Measure your own garage with a tape measure instead of using typical sizes.
- Use the numbers printed on your product labels, manuals and data sheets.
- Change one input at a time to see which assumption matters most.
- Print or save the result and bring it to your installer or contractor.
Next steps
Read the garage ventilation fan guide and the garage dehumidifier guide.
Frequently asked questions
Multiply the garage volume in cubic feet by the air changes per hour you want and divide by 60. It can lower the temperature toward outdoor levels by removing trapped hot air, but it cannot cool below the outdoor temperature. No. The EPA warns not to idle a car inside a garage. Ventilation does not make engine exhaust safe.How many CFM do I need to ventilate a garage?
Will an exhaust fan cool my garage?
Is it safe to run my car in the garage with a fan on?
This tool is for planning and education only. It does not store your entries. Garage door springs and cables are jobs for a trained technician, and new circuits are jobs for a licensed electrician. Read our disclaimer.