How this calculator works
There's no single formula for range hood size. Installers and ventilation guides use a handful of rules of thumb, and each one catches a different way to under-size a hood. The calculator runs all three, takes the highest, then adds for cooking style and duct losses.
1. Cooktop heat or width
Gas: about 100 CFM for every 10,000 BTU of total burner output. A 30 in range with four burners at 18,000, 12,000, 9,500 and 5,000 BTU totals 44,500 BTU, so it needs about 445 CFM.
Electric and induction: about 10 CFM per inch of cooktop width. A 30 in electric cooktop needs about 300 CFM; a 36 in, about 360 CFM.
2. Hood placement
Wall and under-cabinet hoods get help from the wall behind them, which steers smoke up. Island hoods don't, so cross-drafts pull smoke sideways. The rule is about 100 CFM per linear foot of cooktop for wall hoods and 150 CFM per foot for islands. A 36 in (3 ft) island cooktop needs about 450 CFM from this rule alone.
3. Kitchen air changes
The room rule aims to replace the kitchen's air 15 times an hour: length × width × ceiling height × 15 ÷ 60. A 14 × 12 ft kitchen with an 8 ft ceiling holds 1,344 cu ft of air, so it needs about 336 CFM. In open-plan homes, this rule can run high; measure the kitchen area only.
4. Adjustments
- Heavy cooking adds 25% for frequent searing, frying, stir-frying or a grill or griddle burner.
- Duct run adds about 1 CFM per foot of straight duct and 25 CFM per 90° elbow, because resistance cuts real airflow below the hood's rating. Ductless hoods skip this step.
The result is rounded up to the next 50 CFM, which is how hoods are usually rated.
Hood width
A hood should be at least as wide as the cooktop. For gas, heavy cooking and every island hood, go 3 in wider on each side, which usually means the next standard size up: 36 in over a 30 in range, 42 in over a 36 in range. Read more in 30 vs 36 inch range hoods.
Duct size
Duct diameter limits how much air a hood can really move. As a guide, 6 in round ducts suit hoods up to about 400 CFM, 7–8 in up to about 600 CFM, 8 in up to about 900 CFM and 10 in above that. Never shrink the duct below the hood's collar. Use smooth rigid metal duct (the IRC calls for galvanized steel, stainless steel or copper), keep the run short, and end it outside, never in an attic.
Makeup air over 400 CFM
Section M1503.6 of the International Residential Code requires makeup air for any kitchen exhaust system able to move more than 400 CFM. Air leaving the house has to come back in somewhere; without a planned path, a big hood can starve itself and pull exhaust from a water heater or furnace back into the house. Many areas adopt this rule, sometimes with changes, so check with your local building department. See 300 vs 400 vs 600 CFM for how the threshold shapes your choice.
What this calculator can't tell you
- Real airflow depends on the hood's fan curve, filters and duct fittings. Ratings are measured under test conditions.
- Noise matters: compare sone ratings at the speed you'll use most.
- Your manual's clearance and duct instructions override any rule of thumb.
Already have a hood that isn't keeping up? Start with range hood not venting before you buy a bigger one. Deciding how to vent? Read ducted vs ductless and gas vs electric ventilation.
Frequently asked questions
How many CFM do I need for a range hood?
For gas, plan on about 100 CFM per 10,000 BTU of total burner output. For electric or induction, about 10 CFM per inch of cooktop width, so roughly 300 CFM for a 30 inch cooktop. Then check the kitchen air-change and placement rules and use the highest number.
Is 400 CFM enough for a gas stove?
It is enough for a gas cooktop up to about 40,000 BTU total with a short duct run. Many 30 inch gas ranges total 40,000 to 60,000 BTU, which points to 450 to 600 CFM before duct losses.
Can a range hood have too much CFM?
Yes. Oversized hoods are louder, pull heated or cooled air out of the house, and above 400 CFM usually need makeup air under IRC M1503.6. In a tight house an oversized hood can also backdraft other fuel-burning appliances.
Should the hood be wider than the cooktop?
At least as wide, and ideally 3 inches wider on each side for gas, heavy cooking and all island hoods. That is why a 36 inch hood over a 30 inch gas range is a common upgrade.
Does duct length affect CFM?
Yes. Every foot of duct and every elbow adds resistance, so the hood moves less air than its rating. A common rule of thumb adds about 1 CFM per foot of duct and 25 CFM per 90 degree elbow to your target.
Sources
- International Code Council, 2021 International Residential Code, Chapter 15: Exhaust Systems, Sections M1503.3–M1503.6 and M1505.4.4.
- Home Ventilating Institute: certified airflow (CFM) and sound (sone) ratings for residential hoods.
Rules of thumb for general information. Your hood's installation manual and your local building code take priority. Last reviewed September 27, 2026.