Ducted vs recirculating range hood, and duct diameter by airflow
A ducted hood vents outdoors and counts as kitchen ventilation; a recirculating hood only filters and gets no code credit.
What this diagram shows
Two panels. The left green one shows a ducted hood pushing the cooking plume up a smooth metal duct to a roof cap outdoors — it removes heat, steam, and grease and counts as ventilation. The right red one shows a recirculating hood passing air through a charcoal filter and blowing it back into the room, which gets no ventilation credit. A reference bar below maps airflow to duct diameter: up to 600 CFM needs 6 inches, 900 needs 7, 1200 needs 8, and more than 1200 needs 10.
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Range Hood CFM Calculator
Free range hood CFM calculator: size by width or BTU, see it vs the code minimum, and learn if you need makeup air (the 400 CFM rule). No signup.
