I have a 30" induction cooktop and a 30" hood - mine is custom as well with an inset.

I think people are being a bit silly about venting needs since the majority of homes have a microwave vent over the stove which is the same dimensions as the stove or cooktop and probably is LESS effective than a dedicated high quality hood vent.

Also electric and induction do not emit the same pollution as gas does.

I think you are fine doing 30" hood because you are using electric. Sure it is nice to suck up a lot of cooking grease and smell, but the main thing is you have nowhere near the indoor air pollution with electric as you would with gas so no need to worry about it.

It depends on what and how much you cook. If it's basic stuff, with no frying, and very seldom, then 30/30 and no more than 400 CFM is likely fine. If you do challenging cuisine, lots of frying, and every night, then you will want to both go for the 36" and upsize the CFM.

Indoor air quality is not silly. Cooking negatively affects that.

Should be fine. Many cooks don't even turn them on so you're ahead just by doing that.

Ideally, the hood should be 6" wider, but if you rarely cook or don't cook anything with a lot of steam, smoke, or grease, you will probably be OK.

Yes, many do not use their hoods, but that is often b/c they have hoods that are almost useless -- over the range MW hoods, under-powered hoods, improperly installed hoods (usually too high), or recirculating hoods. They often don't "see/smell" any difference so they don't bother using them.

Once you put in a truly functional hood and start using it, you will definitely see a difference and will likely use it every time. That was my experience!

Our hood is 42"W x 24"D over a 36" electric cooktop (as of February, induction!) and installed 30" above the cooking surface -- and I'm glad it's the size it is. When cooking on the side burners, I often see steam rising and expanding enough that if I had a 30" wide hood, it would likely not be caught by the hood.

It's up to you, but I recommend one 6" wider and 24" deep, especially if you can only have a 400cfm exhaust fan.

If you decide to go the 30" route, be sure you do not install it higher than the installation specs state -- usually no more than 30" above the cooking surface.

(Note if on an island or peninsula, then the hood should be at least 6" wider x 27" deep and installed per specs with regards to height above the cooking surface.)

400cfm is not a lot of power. Having a wider capture area will help. You may likely go with more exhaust power if you add a makeup air system.

You will be unlikely to achieve more than 2/3 the rated flow rate no matter what you do, and without a dedicated make-up air path providing low pressure loss replacement air to the kitchen, the actual air flow will depend on how leaky your house is.

To the point: Cooking plumes expand as they rise, and capture of the plume requires that the hood entry area overlap the plume area. Side cabinets can help constrain the plume growth, particularly for back burner cooking. I recommend review of the many hood threads here for more insight.

I did 36 over a 30 induction and am glad I did. I didn’t want an OTR microwave and I bought an inexpensive hood with the idea if I needed to upgrade later, I could. I just wanted it vented to the outside, LED lights, baffle filters and push button vs touch panel/clock controls. It’s only 380cfm but already seems to be WORLDS better than my 1954 mesh filter one speed of unknown power hood that it replaced.

@kaseki Did you once share a point about the heat/btu output of the cooktop being a factor in the acceleration speed of effluent plumes? I might not be recalling this correctly, but I wonder if this means an induction cooktop might ’work’ better (less worse) without the oversizing of the hood than would something like a Bluestar Platinum range with its mega btu heat emission..

First is capture, and this requires plume overlap by the hood. Now it is true that plume divergences vary with different factors, but all need to be overlapped, and few hot grease plumes will stay inside the boundary of the cooktop width except those from the center hobs. Capture is not ameliorated by a lower BTU level at the cooktop surface.

Second is containment. This performs the removal of the captured plumes from the hood entry volume and expels it outside. To avoid reflections on the sheet metal, or at least to contain reflections, a minimum velocity of hood air is needed to entrain the plume elements into the hood flow to get them through the baffles. This varies with the velocity of the plume components (which varies with the heat input to the pan and whether combustion heating is being used) as well as hood design.

So the CFM required for, say, steak searing will be slightly less with induction than with gas, and it might be possible to get away with a nominal 60 CFM/sq. ft. of hood instead of 90 CFM/sq. ft. I would expect that this would still depend on just how hot the searing was, whether the flame was on maximum or we are mostly dealing with a very hot cast iron pan with lowered flame, and how well the hood entry volume (reservoir) was able to average the initial grease vapor transient from throwing the steak into the hot pan.

Without computational fluid dynamics evaluation of a particular case, or schlieren photography of the physical hardware in action, the best I can do is generalize. I have 90 to 100 CFM/sq.ft. and can capture induction wok cooking and induction searing effectively, but the transients are somewhat modest, and my hood is very long with some advantage in reservoir volume. So I know it works for me. What I can't "know" is how every other hood would work for me or for any other cook. Hence, I'll stick with 90 CFM/sq.ft. as a recommendation, with a suspicion that for non-extravagant induction cooking, lower specific flow rates may prove sufficient.

Budgets and space are in many cases limited, so avoid having a zero percent solution because the 100 percent solution is unaffordable or infeasible.