When you consider purchasing a new range hood or replacing an existing one at your home you are confronted by the salespersons or the online order forms with the question about what CFM you need. For many “CFM” is a new and unknown term, and even if you know what it stands for, you most probably don’t know how to determine what CFM range hood you need. You might also wonder what influence the CFM of a range hood could have on the ventilating and cleaning processes of the air in your kitchen.
CFM stands for “cubic feet per minute” and it is the unit the ventilation power of a range hood is measured in. CFM indicates how many cubic feet of air the range hood exhausts every minute at full speed. Thus, the higher the CFM, the more air is vented out of your kitchen or other cooking space. To determine the CFM of the range hood you need, you have to take aspects like the lingering smells and humidity in your kitchen into consideration. The size of your kitchen and the type of cooktop are both playing important roles.
So what CFM range hood do you need? If you have a gas stove your range hood should have 1 CFM per 100 BTU of the cooktop. For an electric stove, the range hood should have 10 CFM per inch width of the cooktop.
To know more about how we reached these numbers, and to know more about range hood CFM calculation, continue reading more.
Kitchen size and cooktop type: 2 important factors to determine the CFM of an indoor hood
The two most important aspects that affect your decision regarding the CFM of your indoor range hood are the size of your kitchen and the size of your electric stove’s cooktop, or if you have a gas stove, the total British Thermal Units (BTUs) of the cooktop. (BTU is a unit of heat defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.)
Generally speaking, the larger the kitchen and the cooktop, the more smoke, and unwanted air have to be cleaned out to keep the air clean and cooking odors at bay. Thus the larger the kitchen and cooktop the higher the CFM of the range hood has to be.
Using your kitchen size to help determine the hood’s CFM
When you use your kitchen’s size as a guideline for the minimum CFM needed for your range hood, you have to calculate the size of the kitchen and then use the cubic feet to calculate the CFM needed for the hood to be effective. It is an easy procedure. All you have to do is to take the following two steps:
Step 1: Measure your kitchen and calculate the volume
Your first step in determining the volume of your kitchen is to measure the floor’s width and length, and the height from the floor to the ceiling in feet. To determine the cubic feet, multiply the width by the length and height.
For example, if your kitchen is 20’ long, 16’ wide, and 9’ tall the size of your kitchen is 20 x 16 x 9 = 2,880 cubic feet.
Step2: Use your kitchen’s volume to calculate the CFM needed
There is a general ventilation rule that a range hood must have the power to exchange the kitchen’s air 15 times per hour. Your second step is now to calculate the number of cubic feet of air to be moved per hour in your kitchen. To do that you just multiply your kitchen’s cubic feet by the air exchanges per hour.
Thus if your kitchen is 2,880 cubic feet as in our example, and you need 15 air changes per hour you have to move 2,880 x 15 = 43,200 cubic feet of air per hour. To determine the CFM needed to enable your range hood to execute this task, divide the cubic feet per hour by 60 to get cubic feet per minute. In our example, you will need at least a 720 CFM range hood for your kitchen to ventilate 43,200 cubic feet of air.
But remember, this is a calculation based only on your kitchen’s size. You still have to compare this with other calculations based on other aspects, like your cooktop’s size or burners.
Use your cooktop’s size or BTU to calculate the CFM needed
If you have a gas stove, you have to find out what is the total British Thermal Unit (BTU) of the cooktop. The BTU of your gas stove might be mentioned in the stove’s manual. If not, you can work on an average of 7,000 BTUs per burner.
Thus, for a 6-burner gas stove, the average total BTUs will be at least 42,000. Divide the total BTUs by 100 to determine the required CFM for your range hood. In this example the CFM needed will be 420.
If you have an electric stove multiply the stove width in inches by 10. So, for a 42’’ electric stove you need a range hood with at least 420 CFM.
The minimum CFM needed based on kitchen and cooktop sizes
After you’ve calculated the CFM needed for your range hood by calculating your kitchen’s size and the size or BTU of your cooktop, take the larger number as the minimum CFM requirement for your new range hood. In our example, the size of the kitchen is the determining factor as it is the largest number. You should look for at least a 720 CFM range hood.
But you have to remember that when you consider more factors to determine the CFM needed for your hood, the number calculated to suit your kitchen and cooktop may be enlarged.
Range hood manufacturers generally recommend that “normal” cooking in a “normal” kitchen with the “normal” number of burners being used simultaneously, requires at least a 600 CFM range hood. That is why it is important to calculate the CFMs needed for the size of your kitchen. Your kitchen might be much larger than a “normal” kitchen! A 600 CFM hood might just not be powerful enough to clean the air in your large kitchen.
Things that determine range hood CFM
Your cooking routines
Your kitchen habits also influence the CFM needed for your range hood. How often you cook, what you cook, and the style you usually use to cook can influence the required CFM.
How often do you cook?
The calculation of a small kitchen’s volume can indicate a rather low CFM for your hood – perhaps even less than 600. But the more frequently you need your stove to prepare food, the more odors can be lingering in your kitchen. Then, even if you have a small kitchen, you’ll probably need at least the recommended 600 CFM range hood to prevent the accumulation of odors. It is also recommended that the hood’s minimum CFM should never be lower than 600 if you usually use all the burners when you are preparing meals.
What do you cook?
If you usually prepare food with strong odors like fish or garlic your range hood has to extract more odors than when you prepare other “normal” types of vegetables and meat. A CFM of at least 600 would be necessary to keep the odors at bay, even if you have a small kitchen.
How do you cook?
Your style of cooking also influences your decision regarding the CFM of your range hood. Generally, you need higher than “normal” CFM range hoods when you love to cook greasy foods, fry foods on high heat, or use a lot of oil.
If you regularly steam your foods or use a cooking method producing a lot of smoke, such as fajitas or pizza, or use a wok often, you will also need a higher CFM hood.
If you are not installing a ductless hood, you have to give a lot of attention to your home’s existing ductwork. Your range hood ductwork has a significant impact on the CFM needed. You need to take the duct length, shape, and size into consideration before deciding on your range hood CFM.
The ductwork is one of the aspects that may add to the CFM number calculated using the kitchen and cooktop sizes.
If your home has existing ductwork, you have to take it into consideration when deciding on your hood’s CFM. It is of no use to have a high CFM hood if the ductwork is too small to handle the air extraction and movement. If, however, you have to install new ductwork, the procedure is turned around. You first determine the necessary CFM for your hood according to other calculations – mainly the kitchen and cooktop sizes – and then fit matching ductwork.
Influence of the ductwork’s diameter
It is important to ensure that the size of the duct matches the ventilation power. Your kitchen exhaust fan can only do its work effectively if the duct is an adequate size. If the duct is too small for your high-CFM hood, the blower will encounter air resistance and smoke and steam and odors will stay in your kitchen.
You can use the following rule of thumb for matching CFM capacity with duct diameter:
- 6” to 8” duct diameter can accommodate 400 to 600 CFM
- 7” to 10” duct diameter can accommodate 700 to 900 CFM
- 8” to 12” duct diameter can accommodate 900 to 1200 CFM
- 10” and up duct diameter can accommodate 1200 and higher CFM
In practice: If, as an example, your existing duct’s diameter is 9” it is of no use to install a range hood with a CFM of more than 900. The duct will just not be able to handle the volume of air from the hood’s fan.
If the situation is the other way round and you have to install a new duct as well, you, for example, have to fit a 10” diameter duct if you are installing a 1200 CFM range hood.
Something to remember: you can never have ductwork that is too wide.
Influence of the ductwork’s length and turns and a roof cap
Apart from the duct diameter, the length of the ductwork also influences the CFM of your hood. If you’ve calculated what CFM you need according to your kitchen and stove sizes you have to increase the CFM by 1 for each foot of ductwork. For each turn in the ductwork, you have to increase the CFM by 25, and you have to increase the CFM by 40 if you have a roof cap.
Let’s look at our example again. If your kitchen size indicates that you need a 720 CFM range hood and you have a 10 feet pipe you have to increase the CFM by 10. For 2 turns in the pipe, you increase the CFM by a further 50, and for your roof cap, you add 40 CFM. In total, you have to increase the original 720 CFM by 100. The minimum CFM for your new hood will thus have to be at least 820.
Is it better to have a higher CFM?
Now that you know how to determine what the minimum CFM for your indoor range hood should be, let’s look at other questions and issues around the CFM of range hoods. A frequent question is whether it is better to have a higher CFM hood.
A higher CFM is better because, generally speaking, it allows you to cook a wider variety of food compared to weaker hoods, while still keeping the air in your kitchen clean and fresh.
But there are circumstances when a higher CFM range hood is not necessarily the best for you.
You don’t need a higher CFM if you have a small kitchen or run your AC/heater often
It is a waste of money to buy a very high CFM range hood when your kitchen is so small that the air pulled into the hood is greater than the air coming into the kitchen. It is also a waste of money if you install a hood with a CFM that cannot be handled by the existing small diameter ductwork.
If you run your air conditioner and heater often the cool and warm air will be extracted from your home if the Hood’s CFM is too high.
A higher CFM is not effective when the duct diameter is too small
The ductwork impacts the capacity of a high-power blower to remove air. If the ductwork diameter is too small your high CFM hood will not be effective. Too long ducts and ducts with a lot of bends will also put a limit on the air volume that can be handled. The money spent on a range hood with a high CFM that cannot fully be utilized is money wasted.
Something to remember
Keep in mind that your range hood’s fan won’t be running at its maximum speed all the time. Most modern range hoods have at least four fan speed options which allow you to adjust the power of the kitchen exhaust fan to meet the specific requirements of the type of food preparation.
Ductless range hoods and their CFM
A ductless range hood is a kitchen exhaust fan that doesn’t take the air in your kitchen to the outside of the house. It recirculates the kitchen air through charcoal filters and doesn’t use ductwork. The charcoal filters trap all the dirt and neutralize odors and then vent the air back into the kitchen.
Although the air is only recirculated ductless range hoods improve your indoor air quality. They are not as effective as ducted range hoods, but it is better than no range hood.
Ductless range hoods can be installed on your wall or over a kitchen island. They are also a cost-effective option for your kitchen because you don’t have the expense of installing ductwork.
Generally, ductless range hoods are good for electric stoves and gas stoves of 36” or smaller. With larger gas stoves too many BTUs are generated for a ductless hood to handle. Compared to ducted range hoods, ductless hoods have a low CFM. Generally, ductless hoods don’t have a CFM of more than 600. On average they are 200-400 CFM hoods.
Usually, recirculating range hoods are sufficient for electric stoves and small gas stoves.
Range Hoods for your backyard
An outdoor range hood is also called a barbecue hood and makes your backyard a place where you can enjoy delicious food and socialize with friends, without odors and smoke all over the area.
Your barbecue hood is a kitchen exhaust fan that is installed outside in an enclosed or open patio. It can be mounted on the wall, or you can hang it from a ceiling away from a wall over a grill island.
It is recommended that you use a barbecue hood with at least 1200 CFM. This will keep the smoke down on your patio and keep it from accumulating on your outdoor kitchen appliances.
Summary: Things to remember about range hoods and CFM
Let’s look again at a few things that are sometimes forgotten when you are deciding on the range hood you want to install.
Range hood size and overhang
The size and position of your range hood are important. The hood has to be at least six inches larger than your range. If not, you can’t maximize the power of your hood fan. For range hoods to work effectively there has to be a proper amount of overhang and depth.
This is particularly true if you have an island range. Be sure your hood has a three to six inches overhang. (If you have an outdoor kitchen, your hood should extend six to 12 inches further than the cooktop.
You will need higher CFM to remove air contaminants properly if your range hood vent is installed more than 36” above the cooktop. If the CFM is too low the hood will struggle to work effectively.
If the range hood is positioned too close to the cooktop, it will also not be able to effectively capture and remove the cooking air. When the hood is too near to the cooktop it can reduce the life of your range hood drastically because of all the heat so near the cooktop.
Ideally, your hood must be installed between 28” to 36” inches away from the cooktop.
The faster your hood fan is running the more noise it is making. If you’re sensitive to sound, you should be aware of this. The fans of range hoods with a low CFM have to run at high speeds to keep the air fresh and clean and thus make a relatively loud noise.
If you have a range hood with a higher CFM it will make less sound as the fan will not run so fast and thus be quieter.
After you’ve done all the calculations regarding your kitchen size, the cooktop’s size and the ducts diameter and bends and have come to the minimum requirements for your range hood’s CFM, it is a good idea to scale it up a bit – but not too much!
Scaling up is very beneficial for island range hoods because they are exposed to your kitchen air on all sides. A wall mount range hood is exposed to air from three sides and the back wall behind your range helps with the funneling of the air through your hood to the outside. Therefore wall mount range hoods don’t need as much CFM as island range hoods to keep you’re your kitchen’s air clean and fresh.
In conclusion, for indoor ducted hoods, determine the hood’s CFM according to the calculations discussed in this article; for ductless hoods install one with the highest CFM (usually about 600 CFM): and for barbecue hoods, install one with 1,200 CFM or higher.