Commercial Kitchen Hood Sizing: CFM Guide by Equipment
IMC requires 200-550 CFM per linear foot depending on hood type and equipment duty level. Here's the complete table with sizing examples.
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Open Calculator →Getting the CFM (cubic feet per minute) wrong on a commercial kitchen hood is expensive no matter which direction you miss. Undersize it and you get smoke, grease, heat, and failed health inspections. Oversize it and you pay for equipment, ductwork, and energy you don't need — plus you create negative pressure problems that make doors hard to open and drafts that affect cooking. This guide covers the engineering basics so you can size your hood correctly the first time.
How CFM Requirements Are Determined
The International Mechanical Code (IMC) and Uniform Mechanical Code (UMC) both specify minimum exhaust rates for commercial kitchen hoods. The calculation is based on:
- Hood type: Wall-mounted (Type I or II), island/peninsula, single or double
- Duty level: Light, medium, heavy, or extra-heavy (based on equipment underneath)
- Hood dimensions: Length and width of the hood opening
The fundamental formula under IMC is:
CFM = Linear Feet of Hood Opening x CFM per Linear Foot
The "CFM per linear foot" rate varies by hood type and duty level. Here are the IMC minimum requirements:
CFM Per Linear Foot by Hood Type and Duty Level
| Hood Type | Light Duty | Medium Duty | Heavy Duty | Extra-Heavy Duty |
|---|---|---|---|---|
| Wall-mounted canopy | 200 CFM/ft | 300 CFM/ft | 400 CFM/ft | 550 CFM/ft |
| Single island canopy | 400 CFM/ft | 500 CFM/ft | 600 CFM/ft | 700 CFM/ft |
| Double island (back-to-back) | 300 CFM/ft | 400 CFM/ft | 500 CFM/ft | 600 CFM/ft |
| Eyebrow/proximity | 250 CFM/ft | 250 CFM/ft | Not applicable | Not applicable |
Note: UMC requirements may differ slightly. Always check your local jurisdiction's adopted code.
Equipment Duty Ratings
Your duty level is determined by the cooking equipment under the hood. This is where most sizing mistakes happen — operators underestimate the duty level of their equipment.
| Duty Level | Equipment Examples |
|---|---|
| Light Duty | Ovens (gas/electric), steamers, kettles, dishwashers (Type II hood) |
| Medium Duty | Griddles, ranges, pasta cookers, convection ovens, tilt skillets |
| Heavy Duty | Charbroilers (under 5 ft), fryers (under 5 ft), wok ranges, high-temp ovens |
| Extra-Heavy Duty | Charbroilers (over 5 ft), upright broilers, salamanders, chain broilers, solid fuel cooking (wood/charcoal) |
Critical rule: If you have mixed equipment under one hood, the entire hood is rated at the highest duty level present. A 12-foot hood with a griddle (medium) and a charbroiler (heavy) is rated heavy duty for the full 12 feet.
Some engineers will segment the hood if equipment is clearly separated, but most jurisdictions apply the highest rating across the board. Confirm with your local AHJ (authority having jurisdiction).
Sizing Example: A Typical Restaurant Kitchen
Let's size a hood for a restaurant with the following cooking line:
- 6-burner gas range (medium duty): 3 feet
- 36" flat griddle (medium duty): 3 feet
- 36" charbroiler (heavy duty): 3 feet
- Double fryer (heavy duty): 2 feet
Total cooking line: 11 feet. Hood length should be cooking line + 6" overhang on each side = 12 feet.
Since heavy-duty equipment is present, the entire hood is rated heavy duty:
12 ft x 400 CFM/ft = 4,800 CFM minimum exhaust
For a wall-mounted canopy hood. If this were an island installation, it would be:
12 ft x 600 CFM/ft = 7,200 CFM minimum
Hood Overhang Requirements
The hood must extend beyond the cooking equipment on all open sides:
- Wall-mounted canopy: 6 inches beyond equipment on front and sides
- Island canopy: 6 inches beyond equipment on all four sides
- Proximity (eyebrow) hoods: Per manufacturer specifications
The depth of the hood (front-to-back) also matters. Standard wall-mounted canopy hoods are 48" or 54" deep. If your equipment line is deeper than the hood, the capture efficiency drops dramatically — the back edge of the cooking surface is too far from the hood filters.
Make-Up Air: The 80% Rule
For every cubic foot of air you exhaust, you need to replace approximately 80% with conditioned make-up air. The remaining 20% is "transfer air" drawn from the dining room and other spaces (which itself must be replaced by the building's HVAC system).
For the 4,800 CFM hood in our example:
- Make-up air required: 4,800 x 0.80 = 3,840 CFM
- Transfer air: 4,800 x 0.20 = 960 CFM
Undersizing make-up air creates negative pressure in the kitchen — doors slam shut or won't open, exhaust efficiency drops because the hood has to fight against the pressure differential, and your HVAC system works overtime trying to compensate. More on this in our dedicated make-up air article.
Common Sizing Mistakes
- Using equipment BTU output instead of duty rating: The code is based on hood type and duty level, not BTU. A 150,000 BTU charbroiler and a 150,000 BTU range have different duty ratings and require different CFM.
- Ignoring future equipment changes: If you might add a charbroiler next year, size the hood for heavy duty now. Retrofitting is 3-5x more expensive than doing it right initially.
- Forgetting the exhaust fan: The hood is just the capture device. You need an exhaust fan sized to deliver the required CFM against your duct system's static pressure. An undersized fan means the hood never reaches its rated CFM.
- Not accounting for duct static pressure: A fan rated at 5,000 CFM at 0" static will deliver far less through 30 feet of ductwork with two elbows. Work with an engineer to calculate actual system static pressure.
When to Hire a Professional
For straightforward single-hood installations (one cooking line, wall-mounted canopy), a knowledgeable equipment dealer or kitchen design firm can handle the sizing. For complex installations — multiple hoods, island configurations, high-rise buildings, or extra-heavy-duty applications — hire a licensed mechanical engineer with commercial kitchen experience. The engineering fee ($2,000-$5,000) is a fraction of the cost of a mis-sized system.
Calculate Your Kitchen Ventilation Requirements
Plug in your hood type, dimensions, equipment list, and duty level to get an instant CFM estimate plus make-up air requirements. Our calculator follows IMC/UMC standards and accounts for all common configurations.