Hood Exhaust Calculator

This calculator sizes a kitchen exhaust hood's airflow the way a mechanical engineer would rough it in: multiply the hood's length by its depth to get the capture area, then apply a CFM-per-square-foot rate keyed to how greasy and hot the equipment underneath actually is — 100 CFM/sq ft for light-duty ovens and steamers up to 250 CFM/sq ft for extra-heavy solid-fuel setups. Island (fully exposed) hoods get a 30% bump on top of that because cross-drafts from all four sides make it harder to capture rising effluent than a hood backed against a wall. The tool then checks that result against a code-minimum CFM-per-linear-foot floor and keeps whichever number is larger, since duty-level area rates and linear-foot minimums diverge for unusually shallow or deep hoods.

From the final rounded CFM (bumped up to the nearest 50 for practical fan selection) it backs into a minimum round duct diameter using the NFPA 96 target velocity of 1,500 feet per minute — grease-laden air needs to move fast enough to stay entrained rather than settling in the ductwork. Static pressure and fan horsepower are then estimated from generic assumptions (clean filters, a 20-foot duct run, typical friction loss) rather than your actual ductwork, so treat those two figures as a starting point for equipment shopping, not a final mechanical schedule. A real system should always be verified by a licensed mechanical engineer against NFPA 96 and your local mechanical code before installation, especially once actual duct length, elbows, and filter loading are known.