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

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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).

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.

The formula to calculate CFM is as follows:

- 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

To calculate CFM (Cubic Feet per Minute), you must know the volume of the room and the ACH.

Use the following formula:

\(\ CFM =\dfrac{Volume\ of\ The\ Room\ (cubic\ ft)\times ACH}{minutes\ in\ an\ hour (60)}\)

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.

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 |

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 |

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 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.

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

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.

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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