The formula
protein = 1.6 g/kg · fat = 25-30 % of calories · carbohydrate = the remainder
Where it comes from
Macros are set in a fixed order for a reason. Protein comes first because the requirement is tied to body weight and to protecting lean mass. Fat comes second because there is a minimum below which hormone production suffers. Carbohydrate takes whatever calories remain, which is why it varies so much between plans.
How to work it out by hand
- Set your daily calorie target from your goal
- Protein: 1.6 to 2.2 g per kilo of body weight, at 4 kcal per gram
- Fat: 25 to 30 % of total calories, at 9 kcal per gram
- Carbohydrate: divide the remaining calories by 4
What is worth knowing
The evidence is much stronger for the protein floor than for any particular split of the rest. Above roughly 1.6 g per kilo the benefit for lean mass flattens out, and going far above it mostly displaces other food. Fat below about 20 % of calories starts to affect hormone production, which is the real floor. Everything after that is preference: low-carb and high-carb plans produce the same result at the same calories and protein, and the one that works is the one you can keep to. Endurance athletes are the genuine exception, because carbohydrate availability limits their performance directly.