CFM (cubic feet per minute) is the amount of air that flows into a room in one minute. It’s usually used to describe how much air exhaust fans or HVAC systems move. Air changes per hour (ACH) is the number of times an air handler cycles through all of the air in a room over the course of an hour. This article will explain how to calculate both ACH and CFM, as well as provide an easy reference table for ACH rates.

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The air changes per hour (ACH) rate is the number of times a minute that the air in your house is exchanged with fresh, outside air. It’s a measure of how well your house’s HVAC system is working and can be used to determine if there are any problems with it.

ACH formula

The formula for calculating ACH can be written as:

ACH = \frac {CFM \times V} {\frac {FV} {60}}


  • CFM stands for cubic feet per minute; this is the amount of air being moved through the ductwork in your home, measured in cubic feet. It’s similar to miles per hour (MPH).
  • V stands for volume, which means “the total amount of space occupied by something,” like all rooms or floors combined together; you would calculate this by multiplying length x width x height and then adding up those numbers together—for example, 6′ x 8′ x 8′=384 sq ft (or 2880 cubic inches).
  • FV stands for “freshly filtered ventilation;” this means how often a room needs to be ventilated out but doesn’t necessarily mean that nobody ever goes into that room again once they’ve been there once before since it depends on who else lives there too! If someone else comes down with allergies after visiting another person’s bedroom then they may feel better when they leave because someone else cleaned up all their dust mites inside their own space without having anything left behind after vacuuming around them. This value will vary depending on what kind of filter system someone has installed throughout different rooms within their house.

CFM meaning

CFM is an abbreviation for cubic feet per minute. It is a unit of measurement for airflow and volume of air. Think of it like you would miles per hour (mph), which measures how fast something goes over a period of time, or miles per gallon (mpg), which measures how much fuel your car uses over a certain distance. CFM measures how much air flows through an opening in one minute.

CFM airflow formula

CFM is the volume of air that passes through a duct or supply line in 1 minute. It’s equal to the product of the pressure difference and the air velocity.

For example, if you have an 800 CFM fan and there is a 2-inch drop from your highest point to your lowest point in one minute, then this would be equivalent to 1200 CFM (800 x 2).

How to use the CFM calculator

The CFM calculator is very simple to use. All you need to do is enter the dimensions of the room, from which it will calculate the surface area and volume of the room. Using the volume, it will calculate the air changes per hour, and the required airflow in different units.


What is a good CFM for a ceiling fan?

4000-5000 is a good range, but going above that, to 6000 is even better.

What is CFM?

CFM is a unit of measurement and it stands for cubic feet per minute.

How many CFM do I need for 1000 square feet?

50 CFM of a continuous ventilation rate.