How Many CFM For My Range Hood? (4 Step Guide)

Do you want to know how much CFM you need for your range hood?

Just a few different ways of calculating range hood CFM include the kitchen size, total BTU output, range hood width, and ductwork length.

In this HomeInspectorSecrets.com guide, you will learn:

how many cfm do i need for my range hood
  • How to choose CFM based on the total BTU output of your range
  • What CFM you need determined by the volume of your kitchen in cubic feet
  • Why the number of turns of your duct should increase CFM
  • How many CFM you need based on the width of your range hood (for electric cooktops)

Let's get started with this guide!

What Is Range Hood CFM?

The meaning of range hood CFM is cubic feet per minute and it is the standard rating for range hoods. This number tells you how powerful the vent hood moves air, and it means how much air volume can be exhausted out of the kitchen in 60-seconds.

The most common CFMs for range hoods are between 200-cfm to 600-cfm but can be as high as 1,000+ for large kitchens and commercial range hoods.

What is a good CFM for a range hood?

Ideally, the best cfm for your kitchen vent hood is powerful enough to adequately remove smoke, odors, and indoor pollutants (such as nitrogen dioxide). But you don't want an overly powerful and unnecessary range hood CFM because it will just add annoying noise and you won't need all of the power (and extra cost).

In addition to this CFM guide, you may want to read my article on range hood building code that discusses duct length code, exhaust cap code, and makeup air code.

Below are the four steps in choosing the best CFM for a range hood:

Step 1. The Best CFM For Electric Cooktops

If you have an electric range or cooktop, you will need to determine the CFM differently than if you have a gas range. Electric stoves typically need less CFM than gas because there is less heat output and less fumes.

Wall Mount & Under Cabinet

If you have a wall mount or under cabinet range hood, we recommend you multiply the linear feet of the hood by 100-cfm. So if you have a 30-inch or 2.5-feet wide range hood, you multiply the width (in feet) by 100-cfm to get 250-cfm minimum.

Island Range Hoods

If you have an island range hood, you will need to use a slightly larger cfm simply because you need more power since it is an open area. Simply multiply the linear feet of the island range hood and multiply it by 150-cfm.

For example, if the island range hood is 3-feet (36-inches), you multiply it by 150-cfm to get 450-cfm minimum.

Read Also >> What Are The Best Ductless Range Hoods?

Step 2. Recommended CFM For Gas Ranges

If you have a gas or propane powered gas range, you will want to use a method that gives you a slightly larger CFM than electric stoves. Gas ranges produce more fumes and more heat than electric cooktops.

The best CFM for gas stoves is to base it on the total burner output BTU of your range.

BTU stands for British thermal unit and it is a measurement of heat. Basically, it is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.

This rule uses 1-cfm per 100-btu of total burner output. The manual or online specs of the manufacturer will list the BTU output of each burner. This CFM calculation will apply to outdoor gas grills as well as indoor.

Here is an example of the BTUs of a 5 burner gas range:

  1. Burner 17,000 BTU
  2. Burner 15,000 BTU
  3. Burner 9,500 BTU
  4. Burner 9,500 BTU
  5. Burner 5,000 BTU

Total Burner BTU = 56,000-btu

56,000-btu / 100 = 560-cfm minimum

If your gas stove has a 56,000 BTU total output for the burners, then you would want a vent hood of at least 560-cfm. A commonly rated 400-cfm range hood would not be enough for a 50k+ BTU range capacity.

Read Also >> What Are The Best Range Hoods By Broan? 

how many cfm for range hood (1)

Step 3. What CFM Based On Kitchen Size

The third way of choosing how many CFM you need will be based on the size of your kitchen. With modern open layouts and tall ceilings, the total size of your kitchen can impact how effective your range hood will be.

In order to determine the best CFM based on your kitchen volume, you need to know that range hoods should exchange or replace your kitchen air once every 4 minutes or 15 times per hour.

To calculate the recommended CFM based on your kitchen size, you simply multiply the height, width, and length of your kitchen in feet in order to get the volume in cubic feet. Once you know your kitchen volume in cubic feet, you simply divide it by 4 (full air exchange every 4-min).

So if you have a kitchen that is 16-ft long, 10-ft wide, and 11-ft tall: 16x10x11 = 1,760 cubic feet. Then take the volume and divide it by four: 1,760 ÷ 4 = 440-cfm. 

440-cfm is the minimum CFM recommended.

If the CFM number is larger than when calculating based on BTU or width, we recommend going with the larger CFM number.

Especially with variable speed range hoods which usually have 3-6 different speeds, you can always use a lower speed and you avoid the mistake of buying an underpowered range hood.

Step 4. Add CFM Based On Ductwork

The size and quality of your duct can have a positive or negative impact on your range hood effectiveness.

The first step is to make sure your duct has the correct diameter for your new range hood. If your duct is too small for the range hood, it may severely restrict airflow and negatively affect the CFM rating. It is pointless to install a 1000-cfm range hood if it only has a 6-inch diameter duct.

To determine the correct duct diameter, read the user manual or spec sheet of the range hood. Range hood duct diameter is usually at minimum 6-inches but may be as high as 10 or 12 inches.

If you are using the correct diameter duct, the next step is to add the recommended CFM below:

  • Add 1 cfm for every foot of duct length. As you can imagine, a very long range hood duct creates more air resistance and you will need more power to overcome this resistance.
  • Add 25-cfm for every turn of the duct. In addition to length, every turn and bend of the duct also adds air resistance and will increase how much CFM you will need.
  • Add 40-cfm for the roof vent. If your range hood goes to the roof, the hood cover will create more air resistance than if it went straight out to the wall.

Read Also >> What Are The Best 400-cfm Rated Range Hoods?

What Are Some Duct Problems That Impact CFM of Kitchen Hoods?

Turns are one of the top ways that your recommended CFM will be reduced with a range vent hood. You want to minimize or eliminate turns altogether—but sometimes that just isn't possible.

If you have two sharp turns in your duct run, add at least a 5-foot section in between each turn. Make sure that the turn uses a reticulated metal elbow so that the airflow smoothly changes direction.

Quality of Ducting

It is highly recommended that you only use smooth metal ducting for your range hood.

If you use foil or corrugated metal ducting, it will reduce performance and it will increase the noise of your stove hood. Grease and debris will catch on corrugated ducting, and may become an eventual fire hazard.

The best ducting is galvanized steel metal ducting because it is the strongest type of duct and it will be the quietest.

Vent Reducer

It is best to always go with the built-in duct size of your stove hood. If you use a vent reducer to adapt to a smaller duct, it will reduce performance, increase the noise, and impact how many CFM you will need.

Vent Cap

The roof or wall vent cap will also have an affect on how much CFM you will need. If the vent cap has a small holed cover, or if it abruptly stops airflow, it will reduce the range hood CFM.

The best range hood caps have gravity closed dampers and doesn't reduce airflow suddenly.

Length of Vent Pipe

The longer it takes for the vented air to get to the outside, the more performance degradation the range hood will experience. The best vent runs are very short such as less than a few feet.

Of course, many times it isn't feasible to have a short vent run such as if you have to go through an attic. If you have the choice, choose the shortest route possible to the outside with the fewest number of turns.

Makeup Air Systems For High CFM Range Hoods

It's important to be aware of potential problems with negative air pressure in a home when using a range hood with high CFM in a small kitchen.

It is recommended that if you install a range hood with 1200 CFM or more and you have a small kitchen, you should provide makeup air. Your local building code may also require a makeup air system even with lower CFMs such as 600-cfm or greater so you should check with your local municipality.

To learn more, I invite you to read my detailed guide on range hood vent code that discusses duct length code, makeup air code, and more.

Makeup air is brought in from the outside to balance the loss of air pushed out by the vent hood. In newer homes, we have made them so energy efficient (tight) that it can be difficult to bring in air from the outside to balance the loss of air from the kitchen hood. In older and leaky homes this isn't likely to be a problem.

Motorized Dampers

A makeup air system is basically a motorized damper that is connected to the range hood. When the range hood turns on, the motorized damper opens, and allows in outside air.

If the negative air pressure problem isn't resolved, it can cause backdrafting of other gas appliances such as the water heater or furnace. Backdrafting is when the exhaust gases get pulled back into the home rather than being pushed outside. This can cause carbon monoxide poisoning in a home.

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