Two numbers, and only one of them is on the fan
Sizing a commercial hood produces an airflow figure, and that figure gets treated as the whole answer. It is half of it. The other half is that the same volume of air has to enter the building, and in a kitchen open fourteen hours a day through a northern winter, conditioning that air costs several times what the fan motor costs to spin.
Run the default case. Ten feet of wall canopy at 300 CFM per foot is 3,000 CFM. The fans, across exhaust and makeup air together, draw about 1.6 kW, which over 5,040 operating hours is roughly 7,900 kWh, or near twelve hundred dollars at fifteen cents. Heating 3,000 CFM by 35 degrees for 2,200 hours a year is about 3,100 therms, which at a dollar thirty is a little over four thousand. So the air is more than three quarters of the bill and the fans are under a quarter of it, and the fans are the part everyone looks at.
This is why two-speed and variable-speed hood controls exist, and why the payback argument for them is made in gas rather than electricity. It is also why an oversized hood is expensive in a way that never appears on the invoice: you pay for the extra airflow every hour the kitchen is open, forever.
Where the CFM per foot figure comes from
The rate is not supplied on this page on purpose. It depends on the hood type — wall canopy, island canopy, backshelf, eyebrow — and on the duty of what is underneath it, which is a real classification: a steam kettle and a solid-fuel charbroiler are not the same problem. The figure that applies to your job comes from the mechanical code your jurisdiction has adopted and from the listing on the hood itself, and listed hoods frequently carry a lower rate than the prescriptive table because they were tested.
What this page does with the rate is geometry. A wall canopy is open along one length, so the rate applies to that length. An island canopy is open along both, so it applies twice, which is why island hoods are so much more expensive to run than people expect. Open ends add the width of the hood to the rated edge on each side that is genuinely open.
Transfer air is not free air
Most kitchens do not supply the whole exhaust from a dedicated makeup air unit. Some fraction is transfer air, drawn out of the dining room, which sounds like it costs nothing because no unit is running to provide it. It costs exactly as much. The dining room now has a deficit, so the building HVAC pulls that much more outdoor air in, and heats it. The bill just arrives on a different line item.
The reason to run some transfer air anyway is that it keeps the kitchen slightly negative with respect to the dining room, so kitchen air moves toward the kitchen rather than the other way. That is a comfort and smell argument, and a good one. It is not a cost argument.
Duct size and the hole through the building
The duct figure here is a free-area calculation at a velocity you choose. Grease duct has a velocity floor, because slow-moving grease-laden air drops its load on the duct wall instead of carrying it to the fan, and it has a practical ceiling set by noise and by fan static. The useful output is usually not the exact diameter but the realisation that 4,000 CFM at 1,800 FPM needs something around twenty inches across going through every floor between the kitchen and the roof.
The residential version of the same problem
If this is a house rather than a restaurant, the arithmetic is smaller but the mechanism is identical, and the range hood makeup air calculator takes the CFM this page produces and asks whether the building envelope can supply it without going dangerously negative. That is the natural next step for a small commercial building too. The duct size calculator and the duct static pressure calculator cover the run itself, and the CFM and air changes calculator handles general room ventilation rather than a capture hood.
Questions people ask
How many CFM does a commercial kitchen hood need?
There is no single figure, and anyone who gives you one without asking what is under the hood is guessing. The rate depends on the hood style and on the duty classification of the appliances, and it is set by the mechanical code your jurisdiction has adopted together with the listing on the hood. A listed hood tested at a lower rate is allowed to use that lower rate, which is most of the reason listed hoods are worth buying. What this page does is take whatever rate applies to your job and turn it into airflow, duct size and an annual cost.
Why does an island hood cost so much more to run?
Because the rate applies along both long sides instead of one. A ten foot wall canopy has ten feet of rated edge; the same hood in the middle of the room has twenty, plus the ends if they are open. The airflow roughly doubles, and so does the makeup air, and so does the gas bill for heating it. That is worth knowing before the kitchen layout is fixed, because moving the cooking line against a wall is free at the drawing stage and impossible afterwards.
Do I need a makeup air unit or can the building supply it?
For a hood of any size in a modern building, the honest answer is usually a dedicated unit, and in many jurisdictions it is required above a threshold the code sets. The reason is not comfort. A building running several thousand CFM of exhaust with no deliberate supply goes negative, doors get hard to open, and any atmospherically vented gas appliance in the building becomes a path for air to come back down. Whether your building needs one, and how it has to be interlocked and tempered, is a question for your mechanical contractor and the inspector, not for a calculator.
Is a variable speed hood control worth the money?
Work it out on gas, not on electricity. The fan saving is real but small. The saving that matters is that you are not heating full airflow during the four hours a day when only one burner is lit. Take the annual figure this page gives, estimate what fraction of open hours actually runs a full line, and you get the number to put against the quote. In a kitchen with a long slow afternoon the case is usually easy; in one that is flat out from open to close it may not be.
Does this tell me whether my hood is code compliant?
No, and it cannot. Compliance depends on the hood listing, the appliance duty classification, the duct construction and clearances, the fire suppression system, the makeup air arrangement and the cleaning schedule, all judged against the edition of the code your jurisdiction adopted and by the official enforcing it. This page is arithmetic on numbers you supply. Take the result to your mechanical contractor and to the plan reviewer.