CFM Calculator
Find the airflow in CFM a room needs for ventilation, from its size, ceiling height, and how many air changes per hour you want.
What your result means
Airflow needed is the cubic feet per minute a fan or vent must move to refresh the room's air at your chosen rate. Room volume is floor area times ceiling height. Air changes per hour (ACH) is how many times the whole room's air is replaced each hour — higher for kitchens and baths, lower for living spaces. Once you have the CFM, size the duct with our duct size calculator.
How to use this calculator
- Enter the room's floor area in square feet.
- Enter the ceiling height (8 ft is standard).
- Pick air changes per hour for the room's use (see the table below).
- Read the required airflow in CFM.
- Choose a fan or duct rated at or above that CFM.
The formula
Room volume times the desired air changes per hour gives cubic feet moved per hour; dividing by 60 converts that to cubic feet per minute (CFM), the way fans and ducts are rated.
Worked example
A 200 sq ft room with 8 ft ceilings holds 1,600 cubic feet. At 6 air changes per hour that's 9,600 cubic feet an hour, or 9,600 / 60 = 160 CFM. Pick a fan or duct rated for at least 160 CFM.
Recommended air changes per hour by room
| Room type | Air changes/hour |
|---|---|
| Living room / bedroom | 4 - 6 |
| Kitchen | 7 - 8 |
| Bathroom | 8 - 10 |
| Laundry / utility | 8 - 9 |
| Garage / workshop | 6 - 8 |
Tips & gotchas
- Bathrooms and kitchens need higher air-change rates to clear moisture and odors.
- Bath fans are often sized at 1 CFM per square foot as a quick minimum.
- Undersized ducts choke a fan — size the duct to the CFM, not the other way around.
- Longer duct runs and more elbows reduce delivered CFM; oversize a little.
- This is a planning estimate — a full ventilation design follows ASHRAE/code.
Frequently asked questions
How do I calculate CFM for a room?
Multiply floor area by ceiling height for volume, then by air changes per hour, and divide by 60. A 200 sq ft room at 6 ACH needs about 160 CFM.
What is ACH?
Air changes per hour — how many times the room's entire air volume is replaced in an hour. Kitchens and baths need more than living spaces.
What CFM do I need for a bathroom fan?
A common rule is 1 CFM per square foot, with a 50 CFM minimum. For larger or higher-ceiling baths, use the air-change method here.
Does ceiling height matter?
Yes. CFM scales with room volume, so a taller ceiling means more air to move for the same number of air changes.
How does CFM relate to duct size?
The duct must carry the required CFM without excess friction. Calculate CFM here, then size the duct with our duct size calculator.
Related calculators
Estimates only — see our full disclaimer.