Kitchen Hood CFM for Gas Stove: How to Calculate Airflow Needs

What CFM Measures in a Range Hood

CFM stands for cubic feet per minute, and it is the standard unit for measuring how much air a range hood can move. A hood rated at 400 CFM can pull 400 cubic feet of air out of the kitchen every minute when running at full speed. The number appears on every hood specification sheet, but it only tells part of the story: the rating is measured under ideal conditions with a short, straight duct, so real-world performance is often lower.

For a gas stove, CFM matters more than it does for an electric cooktop because gas combustion releases moisture, carbon monoxide, and nitrogen dioxide into the air, in addition to the smoke and grease from cooking. A hood that is too weak will leave these pollutants lingering. A hood that is far too strong for the kitchen can create negative air pressure, pulling exhaust gases back down the flue of a furnace or water heater, which is a safety concern in homes with combustion appliances.

The airflow requirement for a gas range is tied to two main factors: the total heat output of the burners, measured in BTUs, and the resistance of the ductwork that carries air outside. Both must be considered together because a hood that is correctly sized for burner output can still underperform if the duct run is long or has too many bends.

The BTU-Based Rule for Minimum CFM

The most widely used guideline for gas stoves is to divide the total BTU output of all burners by 100. A typical gas range with burners totaling 40,000 BTUs would need a hood rated at 400 CFM as a baseline. This rule comes from the Home Ventilating Institute and is intended as a minimum, not a target. If you often cook with several burners on high, you may want more airflow than the calculation suggests.

To use the rule, add up the BTU rating of every burner on the stove. These ratings are listed in the range's manual and often printed on a label near the control panel. A standard five-burner range might have two 5,000-BTU simmer burners, two 10,000-BTU burners, and one 18,000-BTU power burner, for a total of 48,000 BTUs. Dividing by 100 gives 480 CFM, so a hood rated at 500 CFM would be the nearest common size.

This method assumes a standard 8-foot ceiling and a duct run of roughly 10 to 15 feet. It also assumes the hood is installed at the recommended height above the cooktop, which is usually 24 to 30 inches for gas ranges. If your kitchen has a vaulted ceiling or a long duct path, the baseline figure needs to be adjusted upward.

How Duct Run Length Changes the Requirement

Every foot of duct, every bend, and every transition fitting adds resistance that reduces how much air a hood actually moves. A hood rated at 500 CFM at the factory may only deliver 350 CFM through a 20-foot duct with two elbows. To compensate, you calculate the equivalent duct length and add a percentage to the CFM figure derived from the BTU rule.

A common method is to add 1 CFM for every foot of straight duct over the first 9 feet, and 50 CFM for each 90-degree elbow and each roof cap or wall cap. For example, a 20-foot run with two elbows and one wall cap would add 11 CFM for the extra duct length, 100 CFM for the two elbows, and 50 CFM for the cap, for a total adjustment of 161 CFM. Add that to the 480 CFM from the BTU calculation, and you get roughly 640 CFM as the required rating.

Duct diameter also matters. A 6-inch round duct can handle about 400 CFM before airflow becomes noisy and inefficient, while an 8-inch duct can carry roughly 600 to 700 CFM. If your calculation exceeds 400 CFM, an 8-inch duct is generally the right choice. Flexible foil duct creates more resistance than smooth metal pipe, so reduce its length where possible or factor in extra CFM to compensate.

Kitchen Volume and Makeup Air Considerations

The size of the kitchen itself places an upper limit on how much airflow is practical. Building codes in many areas cap range hood airflow at 400 CFM unless the home has a makeup air system that brings in outside air to replace what the hood exhausts. The rationale is that a very powerful hood depressurizes the house, which can interfere with combustion appliances and make doors hard to open.

If your hood is rated above 400 CFM, check local code requirements. A makeup air damper that opens when the hood turns on, or a passive vent connected to the outside, is often required. This is not just a paperwork issue: without makeup air, a 600-CFM hood can pull fumes from a gas water heater's flue into the living space, which is a carbon monoxide hazard. A professional HVAC contractor can test for negative pressure and recommend the right solution.

Kitchen volume by itself does not change the CFM number unless the space is unusually large. The BTU-based rule already assumes a typical home kitchen, and the duct adjustments cover the physical limits of the vent path. If you have a 10-foot ceiling, you may want a slightly higher rating to clear smoke from the taller air column, but the difference is modest compared to the impact of a long duct run.

Practical Sizing Steps for Your Gas Range

Start by writing down the total BTU output of your stove's burners, then divide by 100 to get the baseline CFM. Next, measure the planned duct path from the hood to the outside wall or roof, counting every foot of straight pipe and every fitting. Add the resistance adjustments for elbows, caps, and extra length to the baseline. The final sum is the minimum CFM rating you should look for.

Round up to the nearest commercially available size. Hoods commonly come in 400, 600, and 900 CFM ratings, though some models offer higher. If your calculation lands between sizes, choose the larger one. Also consider the hood's width relative to the stove: a hood that is at least as wide as the cooktop captures more of the rising smoke plume, so a 30-inch gas range is best paired with a 30-inch or wider hood.

Finally, verify the hood's CFM rating is measured at a standard static pressure, usually 0.1 inches of water column. Some manufacturers advertise a higher free-air CFM that drops sharply once ductwork is attached. Look for a rating that specifies the test condition, and compare models on the same basis. If the spec sheet does not say, contact the manufacturer or choose a brand that publishes this detail.