What Size Range Hood Do I Need? Range Hood CFM Calculator
When buying a new range hood for your kitchen, it can be difficult to know how many CFM you really need. CFM stands for cubic feet per minute, which is how much air the hood flows in a given amount of time. CFM happens to be the way most range hoods are measured, and this is important, because this is how quickly the hood will move the poor quality air from your kitchen when you’re cooking. In an effort to make a “Range Hood CFM Calculator” of sorts, we’ll cover some of the major factors that contribute to air flow needs.
It’s important to understand that there are lots of “rules of thumb” circulating around that may or may not be accurate. Rather than promote any of these, the majority of the recommendations here are pulled directly from the Home Ventilation Institute.
Each of these items has a contribution towards the range hood CFM calculation.
Contributors that Require Air Flow
Range Size
Kitchen Volume
Gas Stove
Exhaust Ductwork
According to the HVI, ventilation rates on kitchen range hoods vary greatly depending on where the range is located and what type of cooking is being done. Mounting a range hood direction over the cooking surface is always idea. The funnel or canopy-shaped features on most hoods do a good job of capturing contaminants. For best results, always use a range hood that vents directly outside the home.
Follow along and we’ll detail it all out in an easy to follow step by step process to ensure we get each one accounted for if it applies for you. Keep in mind that these numbers are for hoods against a wall. For range hoods over an island, multiply results by 1.5.
1. The Size of Your Range
The size of your stove or range as it’s called, plays a role in how many CFM are needed for your range hood. For every 12 inches, you’ll need to assume 100 CFM. That means for a 36″ range, you’ll have 36/12 = 3 feet, or a minimum of 300 CFM needed per the HVI recommendation. As you can see, the larger your range, the more CFM that will be required to ventilate the area in your range hood.
2. The Volume in Your Kitchen
The size, and more importantly the volume, of your kitchen plays an important role in any range hood CFM calculation. It’s recommended that the air in your kitchen be exchanged a minimum of 15 times per hour by your range hood. To calculate the volume of your kitchen, multiply the length of the room by the width of the room by the ceiling height.
18ft long x 16ft wide x 9ft ceilings = 2592 cubic feet
To exchange the air every 4 minutes, divide by 4 to get the recommended CFM.
2592 cubic feet / 4 minutes = 648 CFM minimum
3. Are You Using a Gas Stove?
If you’re using a gas stove, understand that they produce more heat than an electric range. This means there will need to be additional air flow capacity on a range hood vent fan above a gas stove. It’s relatively easy to calculate the CFM requirement for a gas stove, assuming you know it’s BTU rating. Assuming you could potentially use all 4 burners at once, you’ll add up the BTU ratings on them to get the total. Average gas burners are roughly 10,000 BTUs each, so we’ll use 40,000 for our example. You should try to determine the actual BTUs of your gas stove.
At minimum, you should have 1 CFM per 100 BTUs. In other words, divide the total number of BTUs to get the minimum recommended CFM air flow for a range hood. In our example above for 40,000 BTU, this stove would require at least 400 CFM.
4. CFM Additions For Ductwork
Since each the length and shape of the ducting adds more resistance to the airflow, some additional will be needed for it to overcome it. It is recommended that when using 8″ diameter smooth metal pipe, you would all 1 CFM per foot of pipe, 25 CFM for each turn, and 40 CFM for a roof cap.
In each of the CFM calculations above, they would be compared to identify which is the worst case scenario and that highest one used. In the case of the ducting, they will be ADDED to the total. So, in a scenario where you need 12ft, 2 turns, and a roof cap, you would be adding:
12ft (12 CFM) + 2 turns (50 CFM) + roof cap (40 CFM) = 102 CFM
Range Hood CFM Calculation
Now it’s time to put it all together and calculate how many CFM you need in your kitchen. Take your recommended CFM numbers from the size of your range, the volume of your kitchen, and the gas stove calculation (if applicable) and compare them. You’ll want to take the worst case scenario, the highest number of these three. In our examples above, that’s #2, the volume of your kitchen at 648 CFM. Now, take #4 from above, the ductwork adder and add it to the number we just selected to get your total recommended range hood CFM calculation. In our example, that would be:
648 CFM (kitchen volume) + 102 CFM (ductwork) = 750 CFM Total Air Flow Recommended
Conclusion
Calculating air flow required by a range hood isn’t that difficult if you know how to do it. Where there are questions or discrepancies, please always follow the guidelines set forth by the range hood manufacturer or check with the HVI for their recommendation, which is always a good idea before taking on any ventilation project in your home. See below for Frequently Asked Questions on calculating range hood CFM requirements and similar topics.
Range Hood CFM Calculator Quick FAQs
We decided to populate this FAQs section to provide answers to common questions related to sizing range hoods. These are all great guidelines, but for any additional questions and/or actual certification, we recommend contacting the HVI.
Is 400 CFM enough for a range hood?
It depends on your range, kitchen size, gas or electric stove top. Please read through our Range Hood CFM Calculator information to confirm.
What should the range hood height above a gas stove be?
When determining the distance between the cooktop and rangehood, the manufacture’s recommendation should always be followed. The typical range often falls between 18″-30″.
How many CFM are needed per BTU on a gas stove?
As shown above, gas stoves will require roughly 1 CFM per 100 BTU. Keep in mind that this is only one portion of the calculation for the required CFM and you need more information (see above) to confirm your needs.
What about island range hoods?
Since island range hoods aren’t against a wall, calculations need to be multiplied by 1.5 in order to have enough air flow.
Standard range hood dimensions seem to be 30″ and 36″ wide. What size range hood do I need?
According to the HVI, “a kitchen range hood should be at least the same width as the cooking surface it will serve and be mounted directly over it”.