Rule of thumb is total BTU divided by 100 equals total CFM. That means if you had a range with 4 18k BTU burners, then you should plan for around 720CFM (18000 x 4 / 100). Of course you can just take the kBTU and multiply by 10 (18 x 4 x 10) to reach the same number which is easier math.

That said, it's just a rule of thumb. Total CFM required depends on the type of hood you use, your install situation, and the type of cooking you do. Long duct runs reduces the net CFM rating of the hood so a 600CFM hood might only be moving 400CFM of air if there is a long duct run or many bends. If you do stirfry, indoor grilling, or heavy saute, that would up your spec a bit as well. Also, nearby windows and doors that are likely to be open during cooking can be a source of drafts that can carry smoke away so they need to be considered as well. Typically, get the most you can afford and can fit in the space. Sometimes getting a larger hood to increase capture area can actually do more than increasing the CFM.

Some would argue that since I never have 2 burners going at the same time that it's ok to have a smaller hood which is reasonable but does reduce your buffer. IMHO the "rule of thumb" already accounts for less than full capacity usage since I doubt 900CFM would be enough if I had 4 full power saute or stirfry pans going. In other words, I would not advise going lower unless you think your usage will be less than "average".

Good luck.

""BUT the Wolf requires 600 CFM""

Recommended, not required. On this cooktop, 440CFM is probably fine. Don't take any shortcuts installing the duct or on the height and it will be just fine. Also, viertical duct is usually more efficient than horizontal.

Although there is a formula as amcook shows, it's typically used for professional style cooking, i.e, a rangetop or dual fuel/gas range.

Stir fry, frying or searing meats often will certainly tax this

I have a Wolf 36" rangetop and went with a 1200 cfm hood. I rarely have it above the halfway mark on the slider control, but I have used it full speed a few times -- like the time I had to pop 120 bags of popcorn (using the burners and the micro to the side) for a school event. I came back home after taking the popcorn to the school. No popcorn smell.

If you don't fry, cook fish, do a lot of stir frying, or pop corn for big crowds or other smelly things, you may be fine with less. I've had less for so long that I enjoy having more available when I need it.

This is slightly OT - but how did you pop bags of popcorn on the rangetop?

In a pan with a lid. The old fashioned way, then scooped it into bags to serve the kiddos.

Oh thanks for answering that - I was trying to figure out if there were some new cookware that enabled you to pop microwave popcorn on a stove took. Glad to know that someone else still remembers how to pop it the old fashioned way. My DD thinks Jiffy Pop is the 'old' way.

Well there is that -- we found Jiffy Pop in the dollar section at Target a year or two ago and had to get some to try it. It was fun to pop, but I don't think it tasted as good as the stuff in the regular pan.