How to Size a Commercial Kitchen Hood

How to Size a Commercial Kitchen Hood: The Sequence

Sizing a commercial kitchen hood follows a fixed order, because each step depends on the one before it. The appliance line sets the hood length and depth, the hood style and duty class set the exhaust airflow, the airflow sets the duct size and fan, and the exhaust total sets the make-up air.

1. List every appliance under the hood with width, depth, fuel and duty class.
2. Lay out the cook line and decide hood style (wall canopy, island, proximity).
3. Add overhang to get hood length and depth.
4. Set the mounting height within the listing and code limits.
5. Determine exhaust CFM from the listed hood data or the code method.
6. Size the grease duct from CFM and target velocity.
7. Size make-up air and decide how it is delivered.
8. Check the result against the adopted code and the AHJ.

Skipping ahead is the most common mistake. An exhaust fan selected before the appliance list is final almost always ends up wrong. If you are new to the physics behind these steps, read how commercial kitchen ventilation works first.

Information You Need Before Sizing a Hood

A hood can only be sized accurately when the cooking equipment and the room are known. Generic assumptions tend to produce either an oversized hood that wastes energy or an undersized one that spills smoke.

Hood Length and Depth From the Appliance Line

Hood length equals the total width of the appliance line plus overhang at each end, and hood depth equals the appliance depth plus front overhang and any space between the equipment and the wall. The overhang gives the expanding plume room to rise into the hood instead of past its edge.

Overhang

Mechanical codes commonly reference a minimum canopy overhang of 6 inches on open sides of the cooking surface, and a listed hood may set its own values. Confirm against the edition adopted locally. Many designers go beyond the minimum at the front and on open ends, particularly over heavy-duty equipment such as charbroilers, because extra overhang improves capture more cheaply than extra airflow.

Depth

Measure appliance depth including knobs, backsplash and flue risers, and add the clearance gap behind the line. Fryers and ranges often have different depths, so take the deepest appliance and confirm the hood front still covers the cooking surface of every unit. On island hoods, overhang applies to all exposed sides.

Practical length checks

• Leave room for equipment to be pulled out for cleaning without leaving the hood footprint.
• Avoid open hood ends right next to a heavy-duty appliance. Move it inward or add an end panel.
• Check whether a long line should be split into two hood sections for fabrication, shipping or rigging.

Setting the Hood Height Above the Cooking Surface

The hood must be close enough to the cooking surface to capture the plume while leaving headroom for cooks to work. The higher a canopy is mounted, the more the plume spreads and mixes with room air before it reaches the hood, which raises the airflow needed to capture it.

The governing limits come from two places. The mechanical code sets maximum distances between the lower edge of a canopy and the cooking surface, and NFPA 96 and the filter listing set minimum distances between grease filters and the cooking surface to protect the filters from flame. A listed hood adds its own installation height range, and that range is part of the tested performance.

In practice the mounting height is usually fixed by three things at once: the listing range, the tallest appliance (for example a salamander or cheese melter mounted on a range riser), and the ceiling height available for the hood body and duct collar. Check all three on a section drawing before ordering the hood.

Exhaust CFM Methods: Code Rates vs Listed Hood Airflow

Exhaust airflow for a commercial kitchen hood is set either by the mechanical code method or by the airflow tested for a listed hood. Both start from duty class and hood style, but they often give different numbers for the same line.

For the same duty class, an island canopy needs more airflow per foot than a wall canopy, and a proximity hood in suitable applications needs less. Duty class pushes the rate up in steps from light to extra-heavy. Whichever method you use, the hood must still capture and contain in a field smoke test. Listed airflow values are only valid when the hood is installed as the listing describes.

Our commercial kitchen exhaust hoods section lists the hood styles we build, and the airflow for each is confirmed on the submittal.

Worked Example: Sizing a 12 ft Wall Canopy (Illustrative Only)

The example below shows the arithmetic of hood sizing. All values are assumptions chosen for illustration. They are not code values and must not be used for a real design without checking the listed hood data and the adopted code.

Assumptions

• Wall-mounted canopy, back against a non-combustible wall.
• Appliance line: 36 in. range, 36 in. griddle, two 18 in. fryers, 24 in. open space with a work top. Total line width 132 in. (11 ft).
• All appliances medium duty.
• Overhang 6 in. at each end.
• Assumed exhaust rate for this example: 300 CFM per linear foot.
• Assumed duct design velocity: 1,500 fpm.
• Make-up air unit supplies 85 percent of exhaust.

A listed hood of the same length could carry a different, often lower, airflow, and that change would ripple through duct size and make-up air. That is why we size from the actual hood submittal, not from a rule of thumb.

Sizing the Grease Duct From CFM and Velocity

Grease duct size comes from one formula: duct area in square feet equals exhaust CFM divided by design velocity in feet per minute. Multiply square feet by 144 to get square inches, then choose a round or rectangular size close to that area.

Area (sq ft) = CFM / velocity (fpm)

Velocity is a balance. A minimum of 500 fpm is commonly referenced to keep grease moving through the duct, and many systems are designed around 1,500 to 1,800 fpm. Confirm against the edition adopted locally. Higher velocity means a smaller duct that fits tight ceilings, but it raises static pressure, fan power and noise.

Check reduced airflow too

If the hood uses demand-controlled ventilation, check velocity at the lowest fan speed as well as at full design airflow. In the example above, running at half airflow (1,800 CFM) through a 2.4 sq ft duct gives 750 fpm, still above the commonly referenced minimum. A duct sized for very low velocity at full flow could drop below it at turndown.

The duct size then feeds the static pressure calculation (hood, filters, duct length, fittings, fan curb) that sets the fan selection. Our grease duct systems page covers material, welding and clearances.

Make-Up Air Quantity and Final Checks

Make-up air must replace the exhaust airflow, with most of it typically delivered by a dedicated unit (often 80 to 90 percent) and the balance arriving as transfer air from the dining room. That keeps the kitchen slightly negative so odors and heat stay out of the front of house.

The quantity is only half of the decision. Delivery matters as much: low-velocity perforated diffusers or perimeter supply keep the plume undisturbed, while fast air near the hood edge can undo a correctly sized hood. Tempering (heating or cooling) depends on climate and kitchen comfort targets. See make-up air systems for unit options.

Before you finalize

• Confirm exhaust CFM against the listed hood submittal, not the early estimate.
• Confirm overhang, mounting height and clearances against the listing and adopted code.
• Check duct velocity at full and reduced airflow.
• Coordinate fan and make-up air unit interlocks and suppression shutdown with the electrical design.
• Plan a smoke test at startup with all appliances hot.

Final exhaust rates come from the listed hood data and the locally adopted code, and the AHJ has the final say. If you want a second set of eyes on a layout, send drawings for a quote and an engineer will review the sizing.