How to Calculate Exhaust Fan CFM Required for a Room?

Ventilation flow in building

Exhaust Fan CFM (Cubic Feet per Minute) is a metric used to measure the airflow capacity of an exhaust fan. CFM represents the volume of air that the fan can move within a minute. It is an important specification to consider when selecting an exhaust fan for various applications, including ventilation systems, industrial settings, bathrooms, kitchens, and other areas where air circulation and removal of contaminants or odours are necessary.

The CFM rating of an exhaust fan indicates the amount of air it can move. A higher CFM value generally means that the fan can move a larger volume of air, resulting in more effective ventilation. Choosing the appropriate CFM rating is crucial based on the specific requirements of the space in which the fan will be installed.

When determining the necessary CFM rating for an exhaust fan, several factors need to be considered:

  1. Room Size: The volume of the room plays a significant role in determining the required CFM rating. A larger space will require a higher CFM to achieve adequate air circulation and ventilation.
  2. Purpose of Ventilation: Different applications have specific ventilation requirements. For example, a bathroom exhaust fan should typically have a higher CFM to remove moisture and odours quickly. A kitchen exhaust fan may require a higher CFM to handle the heat and cooking byproducts effectively.
  3. Air Changes per Hour (ACH): ACH refers to the number of times the entire volume of air within a room is replaced in an hour. Depending on the application, specific ACH values are recommended to maintain air quality. Determining the required CFM can be done by multiplying the room volume by the desired ACH.

Desired Air Changes per Hour (ACH) for specific spaces are listed in below tabular columns.

Air Flow Calculator
Air Changes per Hour

It’s important to note that selecting an exhaust fan with too low of a CFM rating may result in inadequate ventilation and poor air quality. On the other hand, choosing an exhaust fan with an excessively high CFM rating may lead to unnecessary noise, increased energy consumption, and potential discomfort.

Manufacturers provide CFM ratings for their exhaust fans, allowing consumers to select the appropriate fan based on their specific needs. It’s advisable to consult with experts or refer to industry guidelines to determine the recommended CFM rating for a particular application.

In conclusion, the CFM rating of an exhaust fan is a critical specification that indicates the airflow capacity and effectiveness of the fan. By considering factors such as room size, the purpose of ventilation, and desired air changes per hour, one can select an exhaust fan with an appropriate CFM rating to ensure efficient air circulation and ventilation in a given space.

How to calculate CFM for a room?

CFM, or cubic feet per minute, is a unit of measure used for volume flow (usually for fans). To calculate CFM, we have to determine the volume of any room in cubic feet, multiply it by its recommended ACH, and divide everything by 60 minutes per hour. Below is the formula for CFM airflow:

In Imperial System,

Airflow (CFM) = Room Floor Area (Ft) × Ceiling Height (Ft) × Air Changes per Hour (ACH) / 60

In Metric System,

Airflow (m2/s) = Room Floor Area (m) × Ceiling Height (m) × Air Changes per Hour (ACH) / 3600

Air Changes per Hour (ACH)

In any building, there will be a variety of rooms with various functions and purposes. We commonly find a living room, bedroom, dining area, kitchen, and bathroom in a basic house. In most workplaces, we usually have a lobby or receiving area, hallways, function rooms, offices, restrooms, and so much more. Each room has a specific purpose, and we stay in each room for different amounts of time during the day.

Depending on the activities we do in a room, the ventilation required to maintain a fresh air flow will change. Having adequate ventilation and airflow in a room is essential for breathing and for some appliances and equipment to work properly. Aside from that, with correct ventilation, we can control the humidity and temperature in a room and quickly remove any odours, fumes, and even particles that might linger, like in the kitchen or living room. The below table shows the Air Changes per Hour (ACH) required for each type of space

Building/ RoomAir Changes per Hour (ACH)
Living rooms6-8
Laundry room8-9
Business Offices6-8
Conference Rooms8-12
Medical procedure offices9-10
Copy rooms10-12
Main computer rooms10-14
Smoking area13-15
Restaurant Kitchen14-60
Food Staging10-12
Dining Area8-10
Waiting rooms4-8
Hospital wards6-8
Malls and retail stores6-10
Swimming pools10-15
Hospital facilities10-20
Air Changes per Hour required for each type of space

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