[crazy1van]

The weight change = calories in - calories out theory assumes that calories out is some easy to deduce number.

Calorie expenditure can't be determined by just plugging your height, weight, and activity level into an online calculator. The body is more complicated than a bucket with a fixed size hole in it and all you have to do is pour water in at a rate less than it drains out. The body produces hormones to signal to its systems whether to store food. Those hormones are determined by many factors including how much you eat but also what you eat, your genetics, your stress level, how much sleep you get, etc.

[SalmoShalazar]

Yes identifying the exact “calories out” number would be a monumental task. That said, we have a very strong understanding of the ballpark amount of calories it takes to keep your body alive and working for a single day - it’s around 1800, give or take a few hundred calories depending on a few features that you mentioned. With this in mind, it is simple to run a calorie deficit, even if you don’t know exactly how much that number is.