How to use the cfm calculator for room ventilation
- Enter the room length, width and ceiling height in feet.
- Choose Bathroom or Kitchen, or select Custom and type the air changes per hour you want.
- Read the airflow in CFM, the room volume and the same airflow in cubic meters per hour.
- For bathrooms, compare the result with the floor area guideline shown below it and use the larger figure when you shop for a fan.
Formula
Room volume (cu ft) = length x width x height
CFM = volume x air changes per hour / 60
Bathroom floor area guideline: 1 CFM per sq ft, at least 50 CFM, for rooms up to 100 sq ft
1 CFM = 1.699 cubic meters per hour
CFM means cubic feet per minute, the volume of air a fan moves. Air changes per hour (ACH) says how many times the whole room volume is replaced in an hour. The presets are 8 ACH for bathrooms and 15 ACH for kitchens, which are common sizing rules of thumb.
Worked example
A small bathroom is 8 ft by 6 ft with an 8 ft ceiling. Its volume is 8 x 6 x 8 = 384 cubic feet. At 8 air changes per hour the airflow is 384 x 8 / 60 = 51.2 CFM.
The floor area guideline gives 48 sq ft x 1 CFM, which is under the 50 CFM minimum, so a 50 CFM fan is the least to consider. A fan rated at 80 CFM would give comfortable headroom.
A 10 x 12 x 8 ft room at the same 8 air changes needs 128 CFM. At 15 air changes, as for a kitchen, it needs 240 CFM.
What the CFM number is used for
Fans are sold by their CFM rating, so the calculator gives you the figure to look for on the box. Bathroom fans are tested at a stated duct resistance, and long or twisting ducts, small vents and roof caps all lower the real airflow. If the run is long, pick a fan rated well above your minimum.
The airflow figure is also useful for range hoods, workshop fans and whole house ventilation, where you can choose your own air changes per hour for the use of the room.
Choosing air changes per hour
The right number depends on what happens in the room. The presets are starting points, and building codes or manufacturer instructions may require more.
- Bathrooms need to clear steam, so 8 changes per hour is a common target.
- Kitchens create heat, grease and cooking odors, so 15 changes per hour is a common target for whole-room ventilation.
- Rooms with fumes, paint, chemicals or a lot of people may need more, and a professional can advise.
- Quiet matters. A larger fan running slowly is often quieter than a small fan running hard.
Assumptions and limits
- Rooms are treated as simple boxes. Vaulted ceilings, open doorways and connected spaces change real air movement.
- The air change rates are rules of thumb and not code requirements. Check your local building code and the fan instructions.
- Fan ratings are measured under test conditions. Duct length, bends and vent size reduce airflow in a real house.
- The tool does not size makeup air for powerful kitchen hoods.
Frequently asked questions
How many CFM do I need for a bathroom fan?
A common rule is 1 CFM per square foot of floor, with a minimum of 50 CFM, for bathrooms up to 100 square feet. Another is 8 air changes per hour. Calculate both and shop for the larger value, with extra for long ducts.
What does ACH mean?
ACH stands for air changes per hour. It counts how many times a fan would replace all the air in the room in one hour. A higher number means faster ventilation and a more powerful fan.
Is a higher CFM fan always better?
Not always. A very powerful fan can be loud, draw more power and pull air in from places you do not want, such as a fireplace. Choose a rating a little above your calculated need rather than far above it.
How do I convert CFM to cubic meters per hour?
Multiply CFM by 1.699. For example, 100 CFM is about 170 cubic meters per hour. The calculator shows this conversion next to the result.