Look at the BTU rating stamped on your gas cooktop’s spec sheet, add up every burner, and divide by 100. That number is your minimum CFM. A 40,000 BTU gas range needs a hood rated for at least 400 CFM. Match the hood’s width to the cooktop first, then check whether the CFM printed on the box actually holds up once the hood is installed and ducted.
The CFM your cooktop actually calls for
Add up the BTU rating of every burner on a gas cooktop and divide by 100. That is the CFM a range hood needs to pull, according to the Home Ventilating Institute’s (HVI) kitchen ventilation guide. This is industry guidance published by an association, not a number lifted from a building code. Whether your local jurisdiction has its own kitchen ventilation requirement, and what it sets if so, is something only your local building department can confirm, so treat the HVI figure as the sizing floor to design toward, not as proof that any specific installation is compliant.
The HVI figure measures the heat and combustion output a burner puts into the air, not the physical size of the cooktop. A wide cooktop with modest burners can call for less airflow than a narrow one built around a single high-output burner meant for wok cooking or stockpots. That is why the calculation starts with BTU, added burner by burner, rather than with inches.
Take a 30-inch gas cooktop with four burners rated at 9,000, 9,000, 9,000 and 13,000 BTU. Total output comes to 40,000 BTU. Under the HVI rule of roughly 1 CFM per 100 BTU, that cooktop calls for a hood rated at 400 CFM or more. Swap in a single high-output burner rated at 20,000 BTU in place of one of the 9,000 BTU burners and the total climbs to 51,000 BTU, pushing the floor past 500 CFM.
Most gas cooktop spec sheets list a total BTU input for the whole appliance, so you often don’t have to add each burner separately. Check the nameplate or the installation manual rather than estimating from memory. Rounding up from the HVI figure costs nothing at installation and buys headroom if a future range swap brings a higher-output burner into the kitchen.
Electric cooktops don’t carry a BTU rating to add up, which is why they are sized by a completely different rule, covered next.
Gas and electric are sized by different rules
The gas rule and the electric rule aren’t two versions of the same idea. They measure two different things, because a gas burner and an electric element create two different jobs for a hood.
A gas flame throws heat straight up and outward in every direction, and it produces combustion byproducts, carbon dioxide, water vapor, and small amounts of carbon monoxide and nitrogen oxides, that have to leave the kitchen along with the steam and grease. The HVI’s roughly 1 CFM per 100 BTU rule sizes the hood to that combustion load. More burner output means more combustion gas to move, so the CFM figure rises directly with the BTU total, regardless of how wide the cooktop is.
An electric coil or induction element doesn’t burn anything, so there is no combustion byproduct to account for and no BTU rating to add up in the first place. What an electric cooktop produces is steam, grease, and radiant heat rising off a flat surface, and the wider that surface, the more air rises off it. That is why the HVI sizes electric cooktops by width instead of heat output: roughly 10 CFM per linear inch of cooktop width for a wall installation. A 36-inch electric cooktop, on that rule, calls for a hood moving somewhere near 360 CFM.
Because the two rules measure different physical processes, you cannot swap them. Applying the electric width rule to a gas cooktop would ignore how much combustion heat a high-output burner adds to the room. Applying the gas BTU rule to an electric cooktop doesn’t work either, since there is no BTU number to plug in. If a kitchen has a gas range now and the household later switches to electric or induction, the CFM requirement has to be recalculated from scratch with the rule that matches the new appliance, not adjusted from the old number.
A dual-fuel range, gas cooktop paired with an electric oven, is sized by the gas side, since the cooking surface itself is what determines the airflow the hood has to remove.
Why an island hood needs more air than a wall hood
A wall hood has a wall behind the cooktop holding one side of the rising plume of steam, grease and combustion gas in place. An island hood has nothing on any side. Room air currents, a nearby window, someone walking past, an HVAC vent overhead, can push the plume off to one edge before the hood ever pulls it in. That is why the HVI’s kitchen ventilation guide calls for more air over an island cooktop than over a wall cooktop of the same width: the hood is working against open air on four sides instead of three.
Width does part of the job too
The HVI guide doesn’t publish a single fixed multiplier for how much more air an island needs, so there is no one number to plug into the gas or electric rule and adjust upward. What the guide does say is to size the CFM upward from the wall-hood figure for the same cooktop, and to lean on hood width to help capture the plume before it drifts.
The HVI recommends a hood at least as wide as the cooking surface, and 3 inches wider on each side for better capture. On an island, that margin is doing more work than it would against a wall, because there is no wall edge to stop the plume from drifting past the hood’s rim before it clears the room. Mounting height above the cooktop also affects how much of that extra air an island hood actually captures, and that height comes from the range and hood manufacturer’s installation instructions rather than from the HVI’s CFM guidance, so check both documents rather than relying on one.
None of this is building code. It is guidance published by an industry association, the Home Ventilating Institute, and it carries no legal weight on its own in any jurisdiction. Your local building department is the only place to confirm whether a kitchen ventilation requirement applies where you live, and what it specifies if it does.
Reading a hood label: CFM at pressure, not free air
A CFM number on a hood’s box is usually measured with the fan blowing into open air, no duct attached, no elbows, no filter in the way. That is called free-air CFM, and it is close to the best that hood will ever do. Once it’s installed with real ductwork, actual airflow drops, sometimes considerably, because every foot of duct, every elbow, and every grease filter adds resistance the fan has to push against.
What actually changes the number
The figure that matters for real kitchen performance is CFM at static pressure, meaning airflow measured against the resistance of an actual duct run, not open air. Manufacturers that publish this number usually show it on a chart inside the installation manual rather than on the front of the box, so look at the installation manual’s performance chart, not the headline CFM printed on the packaging, before assuming a hood will hit a target figure in a real kitchen.
A few things push real airflow further below the free-air number:
- Long duct runs between the hood and the exterior wall or roof cap
- Multiple 90-degree elbows in the duct path
- Duct diameter that’s undersized for the hood’s blower
- A dirty or clogged grease filter restricting intake
Two hoods with identical free-air CFM ratings can perform very differently once installed, if one is designed around a short straight duct run and the other assumes a longer one with turns. None of this tells you whether a specific installation meets or fails any requirement. It only tells you that the CFM printed on the box and the CFM the hood delivers once ducted are two different numbers, and the second one is the one worth asking a contractor or the manufacturer about directly.
Some hoods bundle their airflow spec sheet with a lighting spec sheet on the same page, since buyers often choose ventilation and task lighting together during a remodel. If the light output is part of the decision too, how many lumens a kitchen needs is a separate question worth checking before settling on a model.
The quiet hood that removes nothing
An undersized hood can run at full speed and still fail to clear the kitchen. It sounds like it’s working, the fan is loud, the light is on, but the plume of steam and grease slides past the edges of the hood instead of rising into it. Over months, that shows up as a sticky film on cabinet faces above the range, a faint grease smell that lingers after cooking, and condensation on nearby windows in cold weather that a working hood should have carried outside.
At resale or renovation, an inspector typically checks life-safety items like duct termination and electrical connections, not the hood’s CFM rating itself, since that figure comes from industry guidance rather than a code checklist. What does show up is a kitchen designer or HVAC contractor flagging the mismatch when a remodel opens the wall and they see the duct size and hood rating side by side. Fixing it then means re-running duct and sometimes resizing the rough opening for a wider hood, work that costs far more once the wall is closed up than it would have during the original install.
If a corrected CFM calculation for a high-output gas cooktop pushes the hood requirement past 400 CFM, the ventilation system needs a way to replace the air being exhausted, a separate consideration covered in make-up air for a range hood over 400 CFM, and one a licensed HVAC contractor should size rather than guess at.
Before buying a hood for a new kitchen or a remodel, run the BTU or width calculation for the actual cooktop, check the hood’s installation manual for real-world CFM at your expected duct run, and confirm any local requirement with the building department. A kitchen designer or the range manufacturer’s technical line can also check the numbers before the hood goes on the wall.
Common questions
Should I just buy the biggest CFM hood I can afford to be safe?
Bigger isn’t automatically better. A hood that moves far more air than the cooktop calls for exhausts more conditioned room air than necessary, and it can reach the point where the ventilation system needs a dedicated way to replace that air. Size to the calculation for your actual cooktop rather than to the largest number on the shelf.
Does the mounting height above my cooktop change the CFM I need?
Mounting height affects how well a hood captures the plume, but the height itself comes from the range and hood manufacturer’s installation instructions, not from the HVI’s CFM formula. Follow both documents rather than assuming one covers the other.
My cooktop has a high-BTU wok burner. Does that change the math?
Yes. Add that burner’s BTU rating into the total along with every other burner before dividing by 100. A single high-output burner can push the CFM floor up significantly compared to a cooktop with only standard burners.
Do these CFM rules apply to a ductless recirculating hood?
The gas and electric rules from the HVI’s kitchen ventilation guide describe airflow that gets exhausted outside. A recirculating hood filters and returns air to the room instead, which is a different kind of system, so these calculations assume a ducted installation rather than a recirculating one.