Ventilation is a bit of a passion for me, but I don't want to confuse you more than you already may be. There are two important factors when choosing your hood:

1. Capture volume - the area inside the hood canopy, above the cooking equipment, to capture heat and smoke as it rises, while the fans remove it.
2. CFM's - cubic feet per minute - the rate of the blower(s) ability to move air.

Lots of things change the equation, like duct diameter, number of turns in the duct before it exits the house, the type of cooking equipment you are using, the height above cooking surface that the hood is mounted, the shape of the hood canopy and more. My general rule of thumb formula, which works in most cases, is:

Length of the hood (in feet) x width of the hood (in feet) x 100 (If against a wall and no stovetop broiler is used)

Length of the hood (in feet) x width of the hood (in feet) x 125 (If against a wall and a stovetop charbroiler is used)

Length of the hood (in feet) x width of the hood (in feet) x 150 (if in a peninsula or island and no stovetop broiler is used)

Length of the hood (in feet) x width of the hood (in feet) x 175 (if in a peninsula or island and a top broiler is used)

Example: Against a wall and over a 36" - 6 burner range, you install a 36" wide by 24" deep hood, then 3' x 2' x 100 = 600 CFM minimum required.

Example: In an island and over a 48" range with 4 burners, 12" grill and 12" charbroiler, you install a 60" wide by 30" deep hood, then 5' x 2 ½' x 175 = 2,187 CFM minimum required. This is usually achieved with a roof-mounted blower.

Again, there are some factors that could reduce or increase the CFM's to do the job, but this formula will work. You should always consult a local HVAC contractor to check for how well sealed the home is, what kind of heating or air conditioning system you have and how the hood may affect it and also if you have a fireplace nearby to be adjusted for.

Remember, your contractor is always welcome to E-mail or call me with questions about ventilation.