What Is a CFM Calculator?
CFM stands for cubic feet per minute, the standard measure of airflow used to size exhaust fans, ventilation systems, HVAC equipment, and air purifiers. This calculator determines how much airflow a space needs based on its volume and the desired number of air changes per hour (ACH). Proper CFM sizing ensures fresh air, controls humidity and odors, and keeps a room comfortable and healthy.
How to Use It
Enter the room's length, width, and ceiling height in feet. Then enter the target air changes per hour (ACH) for the space. Common ACH targets are 6–8 for kitchens, 8–10 for bathrooms, 6–10 for laundry rooms, and 4–6 for living spaces. The calculator multiplies the dimensions to get room volume, then applies the airflow formula to give the CFM rating you should look for when choosing a fan or blower.
The Formula Explained
The calculation has two steps. First, room volume is \(\text{length} \times \text{width} \times \text{height}\). Second, the CFM is given by:
$$\text{CFM} = \frac{\left(\text{Length (ft)} \times \text{Width (ft)} \times \text{Height (ft)}\right) \times \text{ACH}}{60}$$We divide by 60 because ACH is measured per hour while CFM is measured per minute — there are 60 minutes in an hour.
Worked Example
Consider a kitchen that is 12 ft long, 10 ft wide, with an 8 ft ceiling. Volume = \(12 \times 10 \times 8 = 960\) cubic feet. For a kitchen we want 6 air changes per hour.
$$\text{CFM} = \frac{960 \times 6}{60} = \frac{5760}{60} = \textbf{96 CFM}$$So you would choose an exhaust fan rated at roughly 96 CFM or higher.
FAQ
What ACH should I use? It depends on the room. Bathrooms typically need 8 ACH, kitchens 6–8 ACH, and general rooms 4–6 ACH. Check your local building code for required values.
Does ceiling height matter? Yes — taller ceilings increase room volume and therefore the CFM required for the same number of air changes.
Should I round up? Yes. Always choose a fan rated at or above the calculated CFM to account for duct losses and real-world performance.
