Everyday
Range Hood CFM Calculator
Enter your cooktop width and burner output, your kitchen size and duct run, and the page works out the CFM you need, the hood width and duct size to match it, and whether the job triggers a makeup air requirement.
How it works
Behind the numbers
Three methods compete and the largest wins. The width rule gives 100 CFM per foot of cooktop against a wall and 150 per foot over an island, where cross-drafts steal capture. The gas rule divides total burner output by 100, so a 48,000 BTU range needs 480 CFM on burners alone. The volume rule takes the kitchen at 15 air changes per hour, which is volume divided by 4. A 36 inch gas range at 48,000 BTU in a 10 by 14 foot kitchen with 9 foot ceilings scores 360, 480 and 315 on the three rules, so 480 sets the base.
The base then meets the duct. Each 90 degree elbow adds about 10 percent, a run over 15 feet adds about 20 percent, and the two elbows in the worked example lift 480 to 576 CFM, rounded up to a 600 CFM hood to shop for. A ductless hood skips the duct math and takes a 50 percent uplift instead, since the charcoal filter and recirculation cost airflow. Duct size follows the result: 6 inch round up to 400 CFM, 6 to 8 inch to 600, 7 to 10 inch to 900, 8 to 12 inch to 1200, and 10 inch or larger above that.
Hood width and makeup air finish the job. The hood should run 6 inches wider than the cooktop against a wall and 12 inches wider over an island, so a 36 inch range wants a 42 inch wall hood or a 48 inch island hood. Past 400 CFM, section M1503.6 of the International Residential Code calls for makeup air equal to the exhaust rate, and a tight house with a fuel burning appliance is where skipping it causes trouble: the hood can pull combustion gases back down the flue. Local code adoption varies, so the page flags the requirement and leaves the final check to the building department.
FAQ
Questions, answered
How many CFM do I need for a 36-inch gas range?
Start from the burners. A 36 inch range at 48,000 BTU needs 480 CFM by the BTU rule, against 360 by the width rule, so 480 sets the base before duct losses. With two elbows that becomes 576, rounded up to a 600 CFM hood. For typical home cooking, 400 to 600 CFM covers most 36 inch gas ranges once the duct is counted.
How is a gas range hood sized differently from an electric one?
Gas gets a second rule that electric does not need. Burners release combustion byproducts, so total BTU output divided by 100 sets a floor that can beat the width rule. Electric and induction cooktops size on width alone: 100 CFM per foot against a wall, 150 per foot over an island. Kitchen volume still applies to both, and whichever of the applicable rules gives the largest number wins.
What duct size do I need for a 600 CFM range hood?
A 6 to 8 inch round duct, per the common guide table. The bands run 6 inch up to 400 CFM, 6 to 8 inch to 600, 7 to 10 inch to 900, 8 to 12 inch to 1200, and 10 inch or larger above that. The failure to avoid is a 900 CFM hood choked by a 6 inch duct: the fan gets loud and the air never arrives.
Do I need makeup air for a range hood over 400 CFM?
Under section M1503.6 of the International Residential Code, an exhaust system over 400 CFM needs makeup air equal to the exhaust rate. The rule bites hardest in a tight house with a fuel burning appliance, where a strong hood can backdraft a water heater or furnace flue. Some jurisdictions set the line lower or enforce it differently, so confirm with the local building department before finalizing the install.
How many CFM does a ductless range hood need?
About 50 percent more than the ducted figure for the same kitchen. A 300 CFM ducted need becomes 450 CFM ductless, to cover what the charcoal filter and recirculation take. Even then the limits stay: a ductless hood cannot remove heat and humidity the way a ducted hood does, so it fits light cooking better than daily high heat searing.