That's an excellent and very practical question for city dwellers! The short answer is that depth alone isn't the ultimate deterrent; it's a combination of depth, design, and security features. However, focusing on depth, a bin should ideally be deep enough that an animal like a raccoon cannot brace its hind legs on the ground while reaching the bottom. This typically means a minimum internal depth of 30 to 36 inches (about 75 to 90 cm). At this depth, even with their long limbs, most common urban scavengers cannot effectively reach food scraps at the very bottom.
But here's the real-world scoop: a deep bin is useless if the lid is flimsy or unsecured. Scavengers like raccoons are incredibly dexterous and determined. The most effective bins pair sufficient depth with robust, locking lids—often ones that require human hands to open, like a latch or a bar. A deep, smooth-walled interior also prevents them from getting a foothold to climb out if they do fall in.
So, while a good depth creates a crucial physical barrier, think of it as part of a system. The goal is to make the effort of breaking in greater than the potential reward. For truly problematic areas, bins are now designed with weight-sensitive locks or are anchored to the ground. Ultimately, the "deep enough" bin is one that is consistently too frustrating for the local wildlife to bother with, keeping your neighborhood cleaner and safer.