Range Hood CFM: Size for the Burner, Not the Cabinet

The CFM number on a range hood box is measured on a test bench with no duct attached. Your kitchen has six to thirty feet of pipe between fan and outside, and sizing a hood is arithmetic that starts at the cooktop and ends at the roofline.

Start with the burner, not the cabinet

The Home Ventilating Institute (HVI) publishes the sizing method most manufacturers and codes follow. For a gas cooktop, plan on 100 CFM per 10,000 BTU per hour of total burner output — add up every burner rating on the spec plate and divide by 100. A 40,000 BTU/h four-burner range points to 400 CFM; a 60,000 BTU/h pro-style range, 600.

Electric and induction cooktops burn nothing, so the rule switches to width: 100 CFM per linear foot at the wall, 150 over an island.

Then the room check: HVI also recommends exchanging the kitchen's air 15 times per hour — multiply length by width by height, divide by four, take the larger number.

| Situation | Sizing rule | Result | | Gas cooktop, 40,000 BTU/h total | 1 CFM per 100 BTU/h | 400 CFM | | Electric, 30 in wall hood | 100 CFM per linear foot | 250 CFM | | Electric, 30 in island hood | 150 CFM per linear foot | 375 CFM | | 12 x 14 ft kitchen, 9 ft ceilings | 15 air changes per hour | 378 CFM |

A worked example, start to finish

Take a pro-style gas range rated 60,000 BTU/h across its burners. The burner rule gives 600 CFM at the cooktop. The duct plan runs fifteen feet to the roof with two 90-degree elbows, so the penalty math adds about 50 CFM for the elbows (25 each) and about 15 for the run (9 per nine feet). The requirement lands near 665 CFM on paper, which means buying the next standard rating up and sizing the duct diameter for that airflow rather than for the smaller figure.

The last check is the room rule: multiply length by width by ceiling height, divide by four. A hood that clears the burner test usually clears the room test in a standard kitchen, but a large open-plan space can flip the result, and the larger of the two numbers always wins.

Duct runs and elbows eat the margin

Static pressure is the resistance the fan works against, and it comes off delivered airflow. Widely used sizing guidance adds about 25 CFM per 90-degree elbow and 9 CFM per nine feet of duct: a 600-CFM plan with fifteen feet of run and two elbows needs a hood rated near 660 CFM to deliver 600 at the wall.

Material matters as much as length. Smooth rigid galvanized duct is what the fan curves assume; ribbed flexible foil duct catches grease in its ridges and slows air — the most common reason a correctly-sized hood feels weak. Semirigid aluminum flex is the acceptable compromise.

The run should also end outdoors. Dumping hood exhaust into an attic deposits warm, moist, greasy air into the insulation, where the moisture condenses and the grease stays. If the shortest route to outside crosses an attic, route it in rigid duct through the attic and out the roof or gable, not into it.

Recirculating hoods and what they cannot do

Ductless hoods that recirculate through charcoal remove none of the combustion byproducts gas produces — a reasonable trade over electric or induction in a rental, but over gas, ducted works.

The mechanism explains the limit. A charcoal cartridge adsorbs odor molecules until the carbon saturates, then stops adsorbing, and packaging rarely says how long that takes in real cooking. Grease still gets caught by the mesh filter, but steam, moisture, and anything produced by burning gas stay in the room and leave only through ventilation you pay to heat or cool. Recirculating hoods trade a duct run for a recurring filter purchase, and the filter is the part of the spec sheet nobody photographs.

The 400 CFM makeup-air line

In many U.S. jurisdictions a hood above 400 CFM triggers the makeup air damper requirement in IRC M1503.6, since exhausting that much air can backdraft a water heater or furnace.

The mechanism is worth understanding before falling in love with a pro-style range. A hood pulls air out of the house, and the house needs replacement air from somewhere. A leaky older home supplies it through gaps; a tight, well-sealed one supplies it down the chimney of the water heater or furnace, which is how combustion appliances end up venting indoors. The makeup-air damper opens a dedicated path for outside air when the hood runs, breaking that chain. Check the local code position before ordering a high-CFM hood, because retrofitting makeup air after the drywall is up is the hidden price of the big number on the box.

Wall, island, and mounting height

An island hood has no back wall to channel the plume, so add 100 to 200 CFM to the minimum. Standard mounting heights are 24 to 30 inches above an electric cooktop and 30 to 36 above gas, with the manufacturer's clearance taking precedence. Width is the other half of capture: cover the cooktop fully — installers commonly suggest three inches of overhang per side, a 36-inch canopy over a 30-inch range.

The height range exists because the cooking plume widens as it climbs. A hood mounted low catches a narrow column of air; mounted high, it has to pull a wider column the same distance. That is why height and CFM trade against each other: raising the hood to eye level over an island asks the fan to do more work per burner, not less.

Speed and noise

A multi-speed hood earns its rating. Most cooking happens on low: a 600-CFM hood at half speed moves more air, more quietly, than a 300-CFM hood flat out — fan noise climbs faster than airflow. Noise is measured in sones — one sone is roughly a refrigerator's hum — and for an open-plan kitchen, aim for two sones or less at the medium setting you use daily.

Size for the burner, add the duct penalty, respect the height, then let the sone rating decide. Under-cabinet hoods pair with task lighting more than any other style, so read the under-cabinet lighting guide alongside this one; on a gas range, mapping your oven's real temperature covers the heat those burners deliver.