CFM Airflow Calculator

CFM stands for cubic feet per minute, the volume of air a fan or system moves each minute. It is the standard way ventilation and HVAC airflow capacity is rated on product labels.

Multiply the room volume in cubic feet by the air changes per hour, then divide by 60 minutes. A 2,400 cubic foot room at 8 air changes per hour needs 2400 times 8 divided by 60, which is 320 CFM.

Air changes per hour, or ACH, is how many times the room's full volume of air is replaced with fresh air in an hour. An ACH of 8 means the air is swapped out eight times each hour.

It depends on the room. Bathrooms often use 6 to 8, kitchens around 7 to 8, and bedrooms 5 to 6. These are common guidelines, so check your local building code and the fan maker's advice for your space.

Yes. The calculation uses full volume, so a taller ceiling means more air to move and a higher CFM at the same air change rate. That is why height is a separate input rather than assumed.

It is one accepted method. Many bathroom fans are sized at 8 air changes per hour, which this tool computes from the room volume and ACH. A common alternative rule of thumb is 1 CFM per square foot of floor area for smaller baths.

In practice yes. Long duct runs, elbows, and grille resistance lower the airflow a fan actually delivers, so many installers oversize by 10 to 20 percent or use a fan rated for the real duct path. This tool gives the clean airflow target before those losses.

You can, by entering the total conditioned volume, but whole home ventilation often follows a dedicated standard such as ASHRAE 62.2 that factors in floor area and occupants. For a single room or exhaust fan, the ACH method here is the quicker fit.

Yes. The calculation happens in your browser, so the dimensions and air change rate you enter are never sent to or stored on a server.