What CFM of range hood do you need?
For electric range, the minimum CFM of the range hood should be your range width*10. For example, if your cooktop is 36 inches, you will want at least 360 CFM.
Gas ranges, grills, and barbecues are a bit different because they produce a lot of heat and fumes. The amount of heat emitted from a gas cooktop is measured in British thermal or BTUs.If your owner's manual doesn’t tell you the total BTUs of your cooktop, you can find it by adding the BTUs of each burner and then dividing that number by 100. For example, if you have 2 burners that each put out ten thousand BTUs and 2 burners that put out fifteen thousand BTUs. Your total BTUs for your stovetop would be fifty thousand divided by one hundred equals give 5 hundred, so you want a minimum of 500 CFM for your range hood over your gas stove.
Grills and Barbecues have higher BTUs than an indoor gas range but the calculation is the same. Now take the minimum you just got and you have a decision to make. There is a quick way and more precise way to know how many total CFM you are going to need for your new hood. We will cover them both. The quick way, just add 200 CFM to the minimum CFM number you just calculated and you are good to go.
If you want to be a little more thorough, do not just add the extra 200CFM, but just follow these next steps instead. Keep your minimum CFM number handy because we will use it again shortly. Now we are going to factor in the size of your kitchen.
Determine the volume of your kitchen by multiplying its length, width, and height together. For example, if your kitchen is 18 feet long, 14 feet wide, and 9 feet tall, multiply 18 by 14 by 9, and you will get 2,268 cubic feet. Larger kitchens will need to move smoke and unwanted air to keep the air clean and harsh cooking odors at bay. As a rule of ventilation, a range hood needs to have the power to exchange your kitchen’s air 15 times per hour, to get that number, multiply your kitchen’s cubic feet by the air exchanges, for example, 2268 cubic feet by 15 air exchanges equals 34,020 cubic feet per hour. Now divide that by minutes in an hour, 34,020 divided by 60 minutes per hour will give you the CFM needs of your kitchen space, in this example, you will need a minimum of 567 CFM. Compare this number to your minimum CFM number that you calculated earlier, circle the larger of the two.
Then calculate the additional CFM needs of your ductwork. For each foot of ductwork, increase your CFM by one. For each turn in your duct, increase your CFM by 25. And if you use a roof cap, increase your CFM by 40. For example, if your ductwork consists of 12 feet of pipe, 2 turns, and a roof cap, you will add 12 CFM for the 12 feet of pipe, 50 CFM for the 2 turns, and 40 CFM for the roof cap. Now take the CFM number you circled earlier and add your ductwork number to it. In our example, we will add 102, 567 plus 102 equals 669 CFM.
Once you finish this calculation, think about your cooking routine. If you enjoy using your stovetop frequently or do a lot of Asian cooking, frying, barbecuing, boiling, or searing. Add another 200 to the total you have, in our example, we do a lot of searing and frying, we are going to add 200 CFM, which is 669 CFM plus 200 equals 889 total CFM to make shopping for a hood easier round up to the nearest hundred CFM, so we will round up to 900 CFM. That is the more thorough way to calculate the CFM you need for your new hood.