27”. 3” extension in all directions for best capture.

With a 30" deep range, you shouldn't hit your head with a 27" hood. However, they're not as common as 24" and more expensive, as Frieda mentioned. I looked for 27" depths a long time and was almost stuck with ZLine. Luckily, I happened upon a Ventahood that I could afford.

Any hood that is properly sized will extend far beyond 12 inch deep cabinets.

My hood is 27" deep, mounted 36" above range. I'm 5'- 10" and not even close to bumping my head, hood could have even been a couple inches lower without a problem.

alex9179 Im looking at a zLine for $850. Which vent a hood did you go with because ventahoods are pricey.

I found a retailer on Amazon selling the PRXH18-248 (X means 27"). Still pricey but hundreds less than I was expecting, even compared to 36"! It isn't installed but I looked it over and am very happy. The canopy is oversized from my original parameter of 42" but that's a benefit for searing/woking/smashburgers. I don't have upper cabinets to consider, either.

Zline had the best prices but I wasn't really sold on it. Maybe it's snobbery but I was afraid that the $ savings would end up as a regret regarding either power or noise.