Other ones, like your metabolism or overall health, are very difficult to estimate. There are others, like the level of your physical activity, which are more qualitative than quantitative, but it's still possible to measure and classify them. They are quite easy to measure, and you (hopefully) already know some: your age, height, weight, and sex.
The recommended calorie intake, or TDEE (Total Daily Energy Expenditure), depends on many factors.
