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CFM Calculator

Enter values & find how much airflow (CFM) you need for a room or building.

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Airflow Requirements

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This CFM calculator determines the appropriate airflow requirements for a given space, taking into account the room size and recommended air changes per hour (ACH).

What Is CFM?

CFM (Cubic Feet Per Minute) is the unit of measurement that is used to measure the volume of air passing through a given space in one minute. Calculating the optimal airflow is essential for maintaining an HVAC system and for the formation of a comfortable, healthy indoor environment. Nowadays, engineers and HVAC professionals may use CFM calculators to size ductwork appropriately, ensuring efficient air distribution within a building to maintain good air quality.

What Is The Formula To Calculate CFM?

The formula to calculate CFM is as follows:
Airflow (CFM) = Room's floor area × Ceiling height (ft) × ACH / 60where:
  • Room's Floor Area: The room's floor area in square footage
  • Ceiling Height: The height from floor to ceiling(feet)
  • ACH: Air change per hour

How To Calculate CFM?

To calculate CFM (Cubic Feet per Minute), you must know the volume of the room and the ACH. Use the following formula:
\(\ CFM =\dfrac{Volume\ of\ The\ Room\ (cubic\ ft)\times ACH}{minutes\ in\ an\ hour (60)}\)

Air Change Per Hour Formula(ACH):

ACH represents how many times the entire volume of air in a space is exchanged with the new air. Air Changes per Hour (ACH) plays the main role in maintaining a clean and contaminant-free air environment. The Air Change Per Hour Formula is as follows:
\(\ ACH =\ \dfrac{60\times airflow}{\text{room's volume}}\)
For convenience, use a CFM calculator and get the required airflow in CFM to achieve optimal performance from the ventilation system.

Recommended ACH:

Room Recommended ACH
Living room 3 – 4
Bedroom 5 – 6
Kitchen 7 – 8
Bathroom 7 – 8
Laundry room 8 – 9
Attic (if cooling) 12 – 15
Garage (if used for work) 20 – 30

Duct CFM Chart:

The following chart shows the CFM capacity of different-sized square and round ducts:
Cubic Feet Per Min Rectangular Duct Height (in) Round Duct (in)
CFM 4" 6" 8" 10" 12"
50 6x4 5
75 6x4 6
100 8 x 4 6x6 6
125 10x4 6x6 7
150 10x4 8 x 6 7
175 12 x 4 8 x 6 8
200 14x4 8 x 6 8
225 16x4 10x6 8
250 16x4 10x6 9
275 12 x 6 8 x 8 9
300 12 x 6 8 x 8 9
400 14x6 10 x 8 10
500 18 x 6 12 x 8 10 x 10 11
600 20 x 6 14 x 8 12 x 10 12
700 24x6 16x8 12 x 10 12
800 26x6 18 x 8 14x10 12 x 12 13
900 30 x 6 20x8 16x10 12 x 12 14
1000 22 x 8 16x10 14x12 14
1100 24 x 8 18 x 10 16x12 15
1200 26x8 20 x 10 16x12 15
1300 28 x 8 20 x 10 18 x 12 16
1400 30x8 22 x 10 18 x 12 16
1500 24 x 10 20 x 12 16
1600 24 x 10 20 x 12 17
1700 26x10 22 x 12 17
1800 28 x 10 22 x 12 18
1900 30 x 10 22 x 12 18
2000 24 x 12 20

Example:

Calculate the required airflow of a 100 sq. ft. bathroom with 7 ft high ceilings, assuming a typical recommendation of 8 Air Changes per Hour (ACH).

Solution:

Given that: Area = 100 sq. ft Height = 7 ft ACH = 8 Volume = 100 x 7 = 700 cu ft Now use the CFM calculation formula: \(\ CFM =\ \dfrac{Volume\ of\ The\ Room\ (cubic\ ft)\times ACH}{minutes\ in\ an\ hour (60)}\) CFM = (700 × 8) ÷ 60 CFM = (700 × 8) ÷ 60 CFM = 5,600 ÷ 60 = 93.33
For detailed information on calculating CFM, refer to the source fixr.com.

CFM Vs. ACH:

CFM is associated with ACH, but these terms are different in the context of what you want to measure. For instance, the CFM calculates the air volume in cubic feet. If a portable air purifier has a high CFM, then it means it can filter more air and is good for a larger room. This way, it can easily understand how much area a purifier can cover. While ACH(Air changes per hour) is a measure of times that the air is replaced. It helps to understand how often fresh air passes through an area, space, or room.

FAQ’s:

How Much Airflow Is Required For A Room?

The recommended ventilation rate is 135 CFM per square foot of space. So 13.5 CFM would be required for a 100-square-foot area. The amount of airflow required depends on various factors, including:
  • The room's size
  • Use of the space
  • The number of occupants

How Many CFM Do I Need Per Square Foot?

One cfm is needed per square foot (1 cfm/sq ft) of floor area. CFM requirements vary among various rooms and applications based on ventilation needs.

How Much CFM Is Good Airflow?

If the size of the room is under 200 square feet then a good air flow will range from 2000 to 3000 CFM. The value of CFM changes with the size of the space.
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