How to Calculate CFM

Two men taking measurements using a measuring tape
Photo: fotodelux / E+ / Getty Images
Two men taking measurements using a measuring tape
Photo: fotodelux / E+ / Getty Images
Stephanie Mickelson
Written by Stephanie Mickelson
Contributing Writer
Updated Jun 16, 2025
Highlights

  • CFM reflects the measurement of air moving through a space in one minute.

  • To calculate CFM, you need to know the volume of the space and your desired number of air changes per hour. 

  • Your home's HVAC system must match its CFM requirements. 

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Once you know how to calculate CFM, you can make several other HVAC calculations to help maximize the efficiency of the system in your home. This CFM calculator helps you determine how much CFM your space needs based on its volume. 

How Much CFM Is Needed For HVAC 

Technically, CFM stands for cubic feet per minute. A CFM measurement relates to the amount of air that moves through a room in your house. Room CFM varies depending on its volume. 

Choosing a bathroom fan or selecting a ceiling fan is often a matter of deciding what CFM of air the product can move.  

Once you know the volume of a room, you can calculate its CFM based on the number of times per hour you need to change the entire volume of air, which is referred to as ACH (air changes per hour). The average ACH among typical residential rooms is four.

The table below assumes a standard room height of eight feet and an ACH of four.

Room Size (Square Feet)CFM
5026.7
10053.3
200106.7
300160
400213.3
500266.7
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How to Calculate CFM

Calculating CFM for a single room only requires you to know its dimensions and your desired ACH.

CFM = Room Area (sq. ft.) x Ceiling Height (ft.) x ACH / 60

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How to Measure for CFM

You can measure the volume of a simple, rectangle-shaped room by measuring, in feet, its length, width, and height. The length times the width equals its area, while multiplying the area by the height gives you its volume. 

How to Measure Irregularly-Shaped Rooms

To measure the volume of rooms that aren't simple rectangle shapes, treat each section as its own space and add the values together for the room's total volume. For sloped ceilings or angled walls, use the average measurement across the room for your value.

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Who to Hire to Calculate CFM

Having a basic understanding of CFM and how it affects the air quality in your home is useful when reaching out to top HVAC technicians. While knowing how to make the calculations is one part of the equation, a local HVAC contractor also uses several other tools and methods to help maximize your system's efficiency. 

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Frequently Asked Questions

A calculation method that your HVAC contractor will use to ensure your system does what it's supposed to is measuring duct output, or its CFM. The calculation requires knowing the cross-sectional area of the ductwork and determining the air speed past any given point within it.

An anemometer measures the speed at which air travels over or through it. While not a full picture of CFM, an anemometer provides a useful value for helping to determine it. To calculate CFM using one of these tools, you'll need to have a measurement of the cross-sectional area the air is traveling through.

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Learn more about our contributor
Stephanie Mickelson
Written by Stephanie Mickelson
Contributing Writer
Stephanie Mickelson is a freelance writer covering home improvement, real estate, design, and personal finance. Before beginning her freelance career, she worked in residential construction, specifically remodeling kitchens, installing cabinets, flooring, countertops, trim, and more.

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