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
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. 

Get quotes from up to 3 pros!
Enter a zip below and get matched to top-rated pros near you.
Geo Icon

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

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

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.

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. 

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.

Need professional help with your project?
Get quotes from top-rated pros.
The homeowners guide to heating and air conditioning services
From average costs to expert advice, get all the answers you need to get your job done.
Heat pump exterior backyard
Jul 22, 2025
Jul 22, 2025
A heat pump is an eco-friendly heating and cooling system that can save you money on utilities each month. Find out the cost of a heat pump with this guide.
Family playing together on the lounge floor
Apr 14, 2025
Apr 14, 2025
The EPA banned R-22 refrigerant, but if you still have a system that runs on it, here’s the latest on R-22 refrigerant prices for the remaining reserve.
woman, man, and child in cozy blankets huddled next to furnace in low light and smiling
Jul 22, 2025
Jul 22, 2025
When it’s time to replace the furnace, review this guide for new furnace costs based on type, size, and efficiency, plus furnace installation costs.
Woman relaxing in the bathtub
Mar 30, 2025
Mar 30, 2025
Why is your bathroom so hot? It usually points to a ventilation issue, but there may be other factors at play. This guide can help you cool down your space.
A woman cooking on the stove
Mar 25, 2025
Mar 25, 2025
Experiencing a bit of shock and wondering, "Why is my gas bill so high?" The cause could be a number of issues, read on to learn what you can do about it.
ductwork in crawl space in basement
Mar 25, 2025
Mar 25, 2025
What causes condensation on ductwork in crawl spaces? A few things may be to blame. Learn more about the causes and what you can do.