Time Saver: If you’re looking for a quick & easy calculation and already know the principles of CFM and ACH, use the Navigation tool to jump to the Pick HVAC CFM Calculator which gives CFM needed based on the number of ACH your space requires.
OK, for most of us, let’s take it a little slower.
CFM, or Cubic Feet per Minute, is a factor overlooked by some homeowners even though they want good indoor air quality (IAQ) and an HVAC system that adequately and efficiently circulates it.
This page begins with quick answers for those who want the information and need to get on with their day.
Then it digs deeper into the concept of CFM, how it is measured, the importance of ACH (air changes per hour) and, of course, methods for calculating CFM. A CFM and Air Changes per Hour FAQ covers additional ground in this important topic.
- How many CFM do I need?
- CFM Calculator: How Many CFM Do I Need?
- Airflow CFM FAQs
The Pick HVAC CFM Calculator is featured – and it gives you the ACH number you’re looking for. From there, you can determine if your current system is powerful enough to meet the airflow and air changes needs in your home.
How many CFM do I need?
Answer #1: 350 – 400 CFM per ton of HVAC capacity.
That’s the answer to the question: What CFM do I need for my HVAC system to do its job?
This answer of 350-400 cubic feet per minute for each 12,000 BTUs of AC cooling is optimal for the system to run efficiently while adequately cooling and dehumidifying the space.
- 12,000 BTUs = 1 Ton
- 24,000 BTUs = 2 Tons, and so forth
Residential systems range from 1.5 to 5.0 tons, or 18,000 to 60,000 BTUs.
The CFM rating applies to heating too. At 350-400 CFM per 12,000 BTUs of heating capacity, there’s enough airflow to circulate heated air through supply ducts and pull cool air back to the furnace or air handler through the cold air returns.
A 3-ton unit, or 36,000 BTUs, would need a blower capable of moving 3 x 350-400 CFM, so a total of 1,050 to 1,200 CFM.
Answer #2: The CFM needed for a specific room depends on the size of the room and how often you want its air to be changed.
This is the answer to the question: How many CFM do I need to give me the right ACH?
The concept of ACH, or air changes per hour, is totally explained below. And there are step-by-step calculations and examples plus charts.
But here’s a quick example. Let’s say you have a 15 x 20 kitchen with an 8-foot ceiling height, and you’re cooking. Think lots of heat and possibly some moisture vapor from boiling pasta. You want the heat and humidity gone...
Room in cubic feet: 15 x 20 x 8 = 2,400
ACH (air changes per hour) Desired: 10 ACH, or every 6 minutes.
- 2,400 / 6 = 400 CFM
For the kitchen to remain comfortable, the HVAC ductwork would need to deliver 400 CFM of fresh air.
Note that the minutes required for the air to change once is the important number. This is because airflow is in CFM-cubic feet per minute, and you want the air to change every 6 minutes.
If the room is a living room, and 5 ACH is enough, then changing it every 12 minutes would be right (5 x 12 = 60). The equation would be:
- 2,400 / 12 = 200 CFM
All the calculators you find online, including ours, “do the math” for you by converting how many ACH you desire into how many minutes per change.
- 15 ACH = 4 minutes each
- 10 ACH = 6 minutes each
- 8 ACH = 7.5 minutes each, etc.
All this is explained fully as we go.
CFM Calculator: How Many CFM Do I Need?
Let’s walk through this calculator together.
Step 1 – Room Size in Square Feet
Measure the length times the width to get square feet.
If the room is L-shaped or has a bump-out in the middle, then measure and multiply each rectangle separately and add them together.
Step 2 – Ceiling Height
Measure the floor to ceiling. If it is a vaulted ceiling, choose a height about halfway up the slope. That’s the simplest approach, and your result will be accurate.
Step 3 – Air Changes Per Hour (ACH)
Input the ACH.
Step 4 – View the CFM needed
The number of CFM needed to achieve the desired ACH will display automatically.
Again, our CFM Calculator, like those the pros use, converts the number of ACH desired into the minutes it takes per air change and uses the number of minutes an air change takes in the calculation.
How to Calculate Room CFM – An Example
You can check this example with the CFM Calculator above. Try various inputs to see how the data changes as you change room size and necessary ACH – air changes per hour.
Room size: 400 square feet with 10-foot ceilings, or 4,000 cubic feet
Air change frequency: We want the air changed every 12 minutes, so ACH = 5
CFM = 334, according to the Calculator. Is this accurate?
Doing it by hand, we use the CFM formula, CFM = Area x Height x ACH / 60
Plugging in the values, the equation is:
400 (area) x 10 (ceiling height) x 5 (desired ACH) / 60 minutes = 334
CFM – What is It In HVAC?
Cubic feet per minute, or CFM, is a measurement of how much air a fan moves. It addresses the often-overlooked “V” in HVAC – ventilation.
The CFM potential of a blower is determined by the horsepower capacity of the motor and the size of the blower’s fan. The blower fan assemblies are factory designed and tested, and then they are given a CFM rating – meaning they are rated for how many cubic feet (one foot x one foot x one foot) of air they have the ability to push through your ductwork every minute.
Calculating CFM per square foot is determined with this information:
- Size of the HVAC system (1.5 to 5.0 tons)
- Rating of the blower in CFM (Most are 350-400 CFM per ton)
- Square foot of the space served by the HVAC system
The equation is:
System size in tons multiplied by CFM of the blower divided by the number of square feet served.
4-ton AC x 400 CFM blower / 2,000 square feet = 0.8 CFM per square foot.
A room that is 200 square feet would receive 160 CFM
ACH – Determining the number of air changes per hour (ACH) that this system would deliver takes more calculating.
Let’s say the ceiling height is 8 feet. The room is 1,600 cubic feet. At 160 CFM, the air in the room would be changed in 10 minutes. This would produce 6 air changes per hour.
If you wanted more frequent changes, you would have to supply more air to the room along with an additional return vent. More of the system's total CFM would need to be directed to the kitchen, in other words.
CFM Airflow Based on HVAC System Size
The larger the system size – when a properly sized blower is chosen – the more CFM it can deliver. Knowing how many CFM your system is capable of is essential to figuring CFM per room and how long it takes to change the air in those rooms.
|Heating Or Cooling Capacity||CFM (Airflow)|
|How many CFM is a 12,000 BTU (1 ton) unit?||350 - 400 CFM|
|How many CFM is a 18,000 BTU (1.5 ton) unit?||525 - 600 CFM|
|How many CFM is a 24,000 BTU (2 ton) unit?||700 - 800 CFM|
|How many CFM is a 30,000 BTU (2.5 ton) unit?||875 - 1000 CFM|
|How many CFM is a 36,000 BTU (3 ton) unit?||1050 - 1200 CFM|
|How many CFM is a 42,000 BTU (3.5 ton) unit?||1225 - 1400 CFM|
|How many CFM is a 48,000 BTU (4 ton) unit?||1400 - 1600 CFM|
|How many CFM is a 60,000 BTU (5 ton) unit?||1750 - 2000 CFM|
CFM and Common Room Sizes
How many CFM do I need for a 12x12 room? 16x16?
The answer is that it depends on how often you want the air to change. This chart uses room size per square foot rather than dimensions.
It shows how many CFM you need for 2 air changes per hour, or 2 ACH. If you want 4 ACH, multiply CFM needed by 2, and so forth.
|Room Size||CFM (At 2 ACH)|
|How many CFM do I need for 200 square feet?||53 CFM|
|How many CFM do I need for 500 square feet?||133 CFM|
|How many CFM do I need for 1000 square feet?||267 CFM|
|How many CFM do I need for 2000 square feet?||533 CFM|
|How many CFM do I need for 3000 square feet?||800 CFM|
What CFM is Best? High CFM? Low CFM?
The CFM level needs to be “just right” in order for the best indoor climate control and indoor air quality (IAQ).
Is high CFM better? No. There are several negative results of too high CFM. First, do you want a breeze flowing through your house? At times, that might be nice, but generally no, when the heat is just right in winter and the AC temp is perfect in summer, a breeze isn’t appreciated.
What about humidity? Your AC system does more than cool the air by removing heat. It dries the air by removing humidity.
It dehumidifies the air by passing it over the very cold evaporator coil. Moisture in the air condenses on the coil, runs into a drain and is removed from your home.
When air is passing over the coil too quickly, the moisture doesn’t condense out of it. In fact, the airflow might pick up some moisture. The result will be high humidity that will make the air clammy and will potentially lead to mold and mildew in your home.
Is low CFM better? No.
Think about a furnace that makes 100,000 BTUs of heat, but barely moves the air! The heat wouldn’t get circulated, and the rooms near your furnace might be too hot, while the furthest from the furnace would be cold.
In summer, the issue would be reversed.
350-400 CFM per ton is best. Over many years of experimentation and testing, HVAC manufacturers have agreed on this CFM range.
It provides the most efficient and comfortable heating, air conditioning and ventilation for your home.
Indoor Air Quality and Air Changes per Minute
Indoor air can be stale, musty and even polluted.
Some rooms are worse than others – a kitchen with cooking odors and moisture, a home workshop where a table saw is creating dust, or a dining room with 8 chatting people, for example.
These rooms need more airflow – the air needs to be changed more frequently, for example, than in an office occupied by one person. To do this, CFM needs to be higher in those rooms.
ACH, CFM and Ductwork
The ducts in your home must be sized properly to deliver the right CFM of air, so that the ACH number can be what you want it to be.
A 4 inch (4-inch) duct delivers less CFM than a 6 inch duct, for example, which is obvious. See the Ductwork Size and CFM Chart below for details.
Sizing ductwork is one of the most challenging tasks for pro HVAC technicians. It involves computer programs that take into account a lot of data including HVAC system size, blower CFM rating, room sizes, the length of duct runs and much more. “Winging it” DIY isn’t the best option for assuring each room in your home is as comfortable as you want it to be.
Pick HVAC has completed a Complete Guide to Ductwork and will continue to provide resources about this vital, but not well-understood, part of heating, air conditioning and ventilation.
How many CFM per square foot are needed?
As with so much of this information in this CFM Calculator Guide, it depends on the desired ACH.
With the standard 8-foot ceiling, and based on the number of ACH required (shown by ACH = #) and explained below.
- ACH = 1 provides .135 CFM per square foot
- ACH = 2 provides .27 CFM per square foot
- ACH = 3 provides .40 CFM per square foot
- ACH = 4 provides .54 CFM per square foot
- ACH = 5 provides .67 CFM per square foot
- ACH = 6 provides .80 CFM per square foot
- ACH = 7 provides .94 CFM per square foot
- ACH = 8 provides 1.08 CFM per square foot
- ACH = 9 provides 1.22 CFM per square foot
- ACH = 10 provides 1.35 CFM per square foot
What does this mean? If you only need the air changed 1 time per hour (ACH = 1), then just .135 CFM is needed per square foot of space.
So, a 100 square foot room would need 13.5 CFM. A 1,000 square foot room would need 135 CFM.
On the upper end, if you want ACH = 10, or the air changed 10 times per hour, then:
A 100 square foot room would need 135 CFM.
A 1,000 square foot space would need 1,350 CFM
Airflow CFM FAQs
We’ve covered the essentials, but here is additional information, with some overlap.
What types of equipment does CFM apply to?
While we’ve discussed mainly central HVAC systems, fans of all types are rated for CFM.
CFM rating and the desired ACH – air changes per hour – should be considered when shopping for ductless air conditioners and heat pumps, ceiling fans, whole house fans and attic fans, garage heaters, dehumidifiers, air purifiers, room air conditioners such as window and portable ACs, and even simple window/box fans.
Can CFM Be Different Room to Room?
Yes, in fact it needs to be in order to achieve the number of air changes per hour, ACH, desired.
For example, let’s say you have a 150 square foot bedroom and a 150 square foot kitchen. The ACH recommendation for kitchens is double or higher than for bedrooms.
Consequently, you’re going to need more air going into and out of the kitchen than the bedroom.
This will be achieved with larger and/or more supply ducts feeding the room with fresh air and return ducts taking stale air back to the furnace or air handler.
How many CFM are needed to achieve the right ACH?
It depends on a few factors.
Someone might ask, “We have a 200 square foot bedroom, and I like fresh air. How much CFM do we need to change the air five times per hour?”
That’s changing the air every 12 minutes, since 12 x 5 = 60 minutes.
If the ceiling is 8 feet high, the room is 1,600 ft3, or 1,600 cubic feet.
To change the air five times, the system would have to deliver 5 x 1,600 cubic feet per hour, or 8,000.
Since CFM is “per minute,” let’s divide 8,000 by 60 (minutes per hour) to get 133 CFM.
You would need 133 CFM.
Most HVAC equipment and ductwork systems can easily supply that amount of airflow.
What size ductwork do I need?
It depends on the CFM you want to move through them. Here’s a brief duct diameter CFM chart for common duct sizes.
- 4 inch ducts carry 20 CFM
- 6 inch ducts carry 75 CFM
- 8 inch ducts carry 160 CFM
- 10 inch ducts carry 300 CFM
- 12 inch ducts carry 480 CFM
- 14 inch ducts carry 700 CFM
- 16 inch ducts carry 1000 CFM
Important tip: The duct runs together should not greatly exceed the total possible CFM output of the HVAC system – unless you have a zoned system that allows you to mechanically close runs to space/rooms that aren't being used.
Example: You have a 4-ton AC system with a 1,500 CFM blower. The CFM capacity of the ducts, when added together, should be in the 1,500 to 1,700 CFM range.
CFM capability for round metal ducts and rectangular ducts are different, as this interesting chart shows.
Note that metal allows more airflow than flex duct because the interior is smoother. There is less friction, and friction slows air and reduces CFM.