CFM Calculator
Convert room volume and air changes per hour into required CFM.
How this calculator works
CFM = room volume × air changes per hour ÷ 60
Dividing by 60 converts hourly changes into per-minute airflow. Bathroom fans have a separate rule: 1 CFM per square foot of floor, with a 50 CFM minimum, which usually gives a similar answer.
Worked example
With room length 20 ft, room width 10 ft, ceiling height 8 ft, air changes per hour 8 , this calculator returns 213.33 .
Frequently asked questions
How many CFM do I need for a bathroom fan?
One CFM per square foot of floor area, with a 50 CFM minimum, or work from 8 air changes per hour. A 5 by 8 bathroom needs about 50 CFM either way.
What are air changes per hour?
How many times the entire volume of air in a room is replaced each hour. Bathrooms want 8, kitchens 15, living spaces 4, and workshops with fumes 10 or more.
Does duct length affect the fan I need?
Considerably. A fan rated 100 CFM may deliver 60 through 25 ft of flex duct with two elbows. Check the fan curve at the static pressure your run will actually impose.
Can a bathroom fan be too powerful?
In a tight house, yes. A large exhaust fan with no make-up air path can depressurise the building and backdraft a water heater or furnace flue, which is a carbon monoxide risk.