How to Size a Commercial Kitchen Exhaust Hood

What airflow capacity does my exhaust hood need?

Airflow capacity is measured in cubic feet per minute (CFM) and must be high enough to capture and remove heat, grease and smoke at the source. An undersized hood lets contaminants spill into the kitchen. An oversized one wastes energy and can pull conditioned air out of the building too fast.

Engineers use the capture velocity method to calculate the required exhaust rate from the hood type and the heat load underneath. A wall-mounted canopy hood over light-duty equipment such as an oven or steamer needs about 200, 300 CFM per linear foot of hood length. The same hood over heavy-duty charbroilers or wok ranges requires 400, 600 CFM per linear foot. An island hood, exposed on all four sides, needs roughly 50% more airflow than a wall-mounted unit of the same length because it must pull air inward from every direction.

Worked example: a 6-foot wall-mounted canopy over a mixed battery of a fryer and a charbroiler. The charbroiler makes this a heavy-duty application, so use 550 CFM per foot. Multiply 6 feet by 550 CFM to get 3,300 CFM total exhaust. Add 10, 20% for any cross-drafts from open doors or pass-through windows, bringing the target to about 3,600, 4,000 CFM.

How do I measure for the right hood dimensions?

Measure the full width and depth of your cooking equipment, not the cooktop alone. The hood must overhang the appliance by at least 6 inches on all open sides to catch the thermal plume as it rises and spreads. For a 36-inch wide range against a wall, you need a hood that is 48 inches wide if both sides are exposed.

A wall-mounted canopy needs an overhang on only three sides because the wall blocks the rear. An island hood, exposed on all four sides, needs the 6-inch extension on every edge. A 30-inch deep island cooking suite therefore calls for a hood at least 42 inches deep. Extend the overhang to 12 inches for high-output charbroilers or wok stations, where the grease-laden vapour column expands faster.

Height above the cooking surface also affects capture. Mount the hood so its lower edge sits 6.5 to 7 feet above the finished floor, which keeps it within reach of the plume without obstructing the chef's sightline. Standard canopies are 24 to 30 inches deep for wall units and 48 to 60 inches deep for island models, but custom fabrication is common when equipment layouts do not match off-the-shelf sizes.

Which filtration type suits my cooking style?

The filter type inside the hood must match the grease load and heat your cooking equipment produces. A mismatch clogs filters, reduces airflow and raises fire risk. You choose between three main systems: baffle filters, cartridge filters and water wash hoods.

Baffle filters are the standard choice for most hot lines. They work best with solid fuel, charbroilers and woks because the overlapping steel panels spin grease out of the airstream by inertia and tolerate open flame without warping. Cartridge filters use a finer mesh pack to capture smaller particles, so they suit griddles, fryers and combi ovens where the grease aerosol is dense but the flame is contained. Water wash systems inject a detergent spray on a timed cycle to flush grease into a drain. They suit high-volume, 24-hour operations such as hotel banquet kitchens, where you cannot stop production for daily filter cleaning.

Match the filter to the dominant cooking method on your line, not to the exception. If you run a charbroiler next to a fryer, the baffle filter protects the whole hood.

Baffle filters handle solid fuel, charbroilers and woks where open flame and large grease droplets are common.
Baffle filters are the easiest to maintain. You run them through a dishwasher and they have no replaceable media.
Cartridge filters capture finer grease mist from griddles, fryers and combi ovens without open flame.
Cartridge filters need regular replacement of the mesh pack when pressure drop rises, typically every few months under heavy use.
Water wash systems suit 24 hour operations like hotel and institutional kitchens that cannot pause for manual cleaning.
Water wash hoods require a hot water supply, detergent injection and a floor drain connected to a grease trap.

What ductwork and makeup air factors affect performance?

The duct diameter and total length directly control static pressure, which the exhaust fan must overcome. A duct that is too narrow or has too many bends forces the fan to work harder and reduces the effective CFM at the hood. For a typical Type I hood over heavy cooking equipment, a duct velocity of 1500 to 2000 feet per minute is the design target. Every 90-degree elbow adds the equivalent of 10 to 15 feet of straight duct to the static pressure calculation.

Makeup air replaces the conditioned air pulled out of the kitchen. A common rule is to supply about 80% of the exhausted air volume through a dedicated makeup air unit, leaving a slight negative pressure in the kitchen. This prevents cooking odours and heat from spilling into the dining room. Without controlled makeup air, the hood pulls air from door gaps. This causes drafts and makes the exhaust fan run outside its performance curve.

Keep duct runs as short and straight as the building layout allows.
Use welded grease duct with a minimum slope of ¼ inch per foot toward the hood or a grease trap.
Match the duct diameter to the hood collar size and the fan inlet to avoid bottlenecks.
Position makeup air diffusers away from the hood capture zone so they do not blow grease-laden air back into the kitchen.
Balance the system with a commissioning test after installation, not just on paper.

How do fire suppression requirements influence hood selection?

Your cooking equipment determines whether you need a Type I or Type II hood. Type I hoods carry a listed fire suppression system and are mandatory over appliances that produce grease and smoke, such as fryers, griddles, charbroilers and woks. Type II hoods handle heat and steam only, with no fire suppression, and suit dishwashers, steam kettles and pasta cookers.

UL 300 compliant systems are the current standard for all new installations. They discharge a wet chemical agent that saponifies hot grease, forming a soapy blanket that smothers the flame and prevents reflash. The nozzle layout inside the hood canopy is engineered to cover specific appliance hazard zones, so the hood manufacturer matches the distribution piping to the equipment lineup beneath it. A fryer bank requires nozzles positioned directly above each vat, while a charbroiler needs wider coverage to protect the radiant surfaces.

Verify that the hood listing plate matches your cooking battery before ordering.
Check that the suppression system nozzles align with the appliance cut sheets provided by the manufacturer.
Confirm the hood depth extends far enough to capture the full plume from tilted skillets and open flame equipment.
Ensure the fire system manual pull station will be mounted along the exit path, not blocked by shelving.
Schedule a semi-annual service with a licensed fire protection contractor to maintain UL 300 compliance.

What stainless steel grade and gauge should I look for?

Type 304 stainless steel is the standard for commercial exhaust hoods. It resists corrosion from heat, grease and the acidic cleaners used in professional kitchens. Type 430 costs less but contains no nickel, so it will eventually show rust in humid or salty environments. If the hood is installed above a dishwasher or in a coastal kitchen, 304 is the minimum you should accept.

Gauge refers to the thickness of the steel sheet, and a lower number means thicker material. Most commercial hoods use 18-gauge or 20-gauge for the canopy body and 16-gauge for the removable grease filters. Thicker steel resists denting during cleaning and vibrates less when the fan runs at full speed. This directly reduces the low-frequency hum that staff complain about in tight pass-through areas.

304 stainless for the hood shell and exposed surfaces in any kitchen with heavy steam or acidic marinades.
430 stainless can work for the internal plenum only if the manufacturer isolates it from moisture contact.
18-gauge (1.2 mm) canopy panels keep the hood rigid under high airflow without adding excessive weight.
20-gauge (0.9 mm) is acceptable for light-duty hoods over equipment that produces minimal grease.
16-gauge (1.5 mm) filter frames hold their shape through repeated removal and dishwasher cycles.
A heavier gauge does not improve extraction performance but it cuts noise transmitted through the metal by roughly half compared to 22-gauge.

FAQ

Can I install a shorter hood to save money?

A hood that does not extend 6 to 12 inches past the cooking surface on open sides will miss the thermal plume at the edges. Grease and heat escape into the kitchen, raising cooling costs and fire risk. The savings disappear quickly in higher HVAC loads and cleaning expenses.

How often should I clean the exhaust hood and filters?

Baffle filters need washing every one to two weeks in a busy kitchen. Hood interiors and duct access points require professional cleaning quarterly or per local fire code. Neglected filters reduce airflow and force the fan to work harder, which shortens motor life.

Does a higher CFM rating always mean better ventilation?

No. Oversized exhaust pulls more conditioned air out of the kitchen and raises makeup air costs. A properly sized hood captures the cooking plume at the lowest effective airflow, which keeps energy bills under control and reduces noise.