How to calculate CFM

CFM, or cubic feet per minute, is a unit of measure used for volume flow (usually for fans)

Room air change per hour requirements

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 odors, fumes, and even particles that might linger, like in the kitchen or living room.

In order to remove these air pollutants, we have to change the air in the room by introducing new volumes of air. Depending on the room, we might need to perform multiple air changes per hour to achieve our desired air quality in that room

We consider one air change per hour or 1 ACH to occur when an entire room’s air volume is replaced once with new air within an hour. As an example, a 30 m³ room that requires four air changes per hour would need 30m³ × 4 = 120 m³ of air to flow in it within an hour.

What is CFM? 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 for 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:

airflow = room's floor area × ceiling height (ft) × ACH / 60

where:

• airflow – Required air volume flow rate across the room in the CFM unit;
• room's floor area – The room’s floor square footage;
• ceiling height – The room’s average height from floor to ceiling in feet; and
• ACH – Air change per hour required for the given room.

Air changes per hour formula

As you already know what CFM means, you shouldn’t have any problems evaluating its corresponding parameter, ACH.

If in case you need to determine the air change rate of a room, provided that you know its volume and airflow, we can rearrange the formula for CFM airflow to derive the ACH formula, as expressed below:

ACH = 60 × airflow / room's volume

Please note that the room’s volume must also be in cubic feet and the airflow is also in cubic feet per minute.

Sample on how to calculate airflow in CFM

Let’s say we want to install a ventilation system that would provide 8 ACH to a 250 ft² home kitchen with a ceiling height of 8 ft. Using the formula for CFM airflow, we can estimate the required CFM for the kitchen, as shown below:

airflow (CFM) = floor area × ceiling height × ACH / 60

airflow = 250 ft³ × 8 ft × 8 ACH / 60

airflow = 2000 ft³ × 8 ACH / 60

airflow = 266.6666 CFM ≈ 266.67 CFM

We now know that we have to install a ventilation system that could generate roughly 270 CFM for the said kitchen from the calculation above. It would be best to work out the CFM requirement for each room. This way, we can tell if we would need a robust ventilation system to supply adequate airflow to each room.

It is important to note that this is a general formula. The actual CFM requirements to ventilate a room may vary depending on a number of factors, such as the number of people in the room, the activities taking place in the room, and the type of ventilation system in the room. If you are unsure how many CFM you need, it is always best to consult with a professional.

Additional factors to consider when calculating CFM requirements

• The number of people in the room. The more people in the room, the more CFM you will need to remove the stale air and replace it with fresh air.
• The activities taking place in the room. Activities that generate heat, such as cooking or exercising, will require more CFM to remove the hot air and replace it with cooler air.
• The type of ventilation system in the room. If the room has a natural ventilation system, such as open windows or doors, you will need less CFM than if the room has a mechanical ventilation system, such as an air conditioner or furnace.
• Type of filter being used. CARBON filters and HEPA filters have different CFM requirements, make sure you check the specs closely 

By considering all of these factors, you can determine the correct CFM requirements for your room and ensure that you have the proper ventilation to keep the air fresh, healthy and clean.

This CFM calculator will help you determine the required air flow rate in CFM units for any room square footage and air change per hours, or ACH.