What I've read is the hood should be about 6" wider than the cooktop so you get 3" extra on each side. I'm buying a 30" induction cooktop, so I'm getting a 36" hood. You should get a 42" hood.

The other thing I've been told here is that the rule of thumb for ventilation over an induction cooktop is 100 CFM for every foot of cooktop, so you want at least 300 CFM.

I'll get a hood somewhere between 400 and 450, just to give me a little wiggle room and because that's already a pretty low CFM hood.

I don't use length as a criterion for flow rate, as required flow is better determined by hood aperture area and thermal plume velocity, but as I recall the rule of thumb, which was originally commercial, it was based on length of hood boundary, so for island hoods it would be the twice the length plus twice the front-to-back depth times 100 cfm. For wall hoods I think the length was only counted once plus the two sides. If side skirts were used then only the length.

(Note: Commercial hoods have thin sides, so hood aperture is essentially the hood dimensions. Residential hoods may have apertures that are smaller than the hood dimensions. It is aperture that counts for flow rate, but note further that if the aperture does not extend to the limits of the expanding effluent cone, then some of the effluent will not be captured, much less contained.)

For rules of thumb that have some cachet, there is a kitchen ventilation guide at Greenheck.com that has some rules. A link may be found in my Clippings, or rummaging around at Greenheck may be beneficial.

kas