Split your daily calorie target into protein, carb and fat grams.
Protein and carbs = 4 kcal/gram, fat = 9 kcal/gram. Grams = (calories × percentage) ÷ kcal-per-gram.
For general wellness information only — not medical advice. Consult a healthcare professional for personal guidance.
Splitting calories into round percentages like "40% protein, 30% carbs, 30% fat" is a convenient shortcut, but it puts the cart before the horse: your muscles don't respond to a percentage, they respond to grams of protein actually consumed, so protein should be set first in grams per kilogram of body weight, and the percentage split should fall out of that — not the other way around. Evidence-based ranges for someone training regularly run roughly 1.6-2.2 g/kg of body weight (about 0.7-1g per pound) whether the goal is muscle gain or fat loss with muscle preservation; a 70kg person training seriously should be targeting 110-155g of protein a day regardless of what percentage that happens to work out to.
Once protein is fixed in grams, fat should be set next — keep it at roughly 20-35% of total calories, and don't go meaningfully below about 0.6g per kg of body weight for extended periods, since dietary fat supports hormone production. Carbohydrates fill whatever calories remain after protein and fat are accounted for, which is why carb targets swing the most between goals: a bulking phase typically runs carb-heavy (fuelling training volume) while a cut leans on the higher end of the protein range with carbs trimmed to create the deficit. At the raw energy level, protein and carbohydrate both provide 4 calories per gram, while fat provides 9 — more than double per gram — which is the arithmetic reason fat percentages look small even at a nutritionally adequate gram intake.
Muscle protein synthesis responds to an absolute amount of protein consumed, not a percentage of your calorie intake. Setting protein in g/kg of body weight first (typically 1.6-2.2 g/kg for someone training) and letting carbs and fat fill the remaining calories gives a more physiologically meaningful target than starting from a round percentage split.
Protein and carbohydrate both provide 4 calories per gram; fat provides 9 calories per gram — more than double. This is why fat can look like a "small" percentage of your macro split even when the gram amount is nutritionally adequate; it's simply a denser source of calories per gram.
Protein is set from body weight and stays relatively stable regardless of goal, and fat is kept within a fairly narrow 20-35% band for hormonal health. Carbohydrates absorb whatever calories are left after those two are fixed, so they swing the most — higher during a muscle-building surplus to fuel training volume, lower during a fat-loss deficit.
Yes — staying below roughly 0.6g of fat per kg of body weight for extended periods can affect hormone production, since dietary fat is a building block for several hormones. Even in an aggressive calorie deficit, it's worth keeping fat intake at or above that floor rather than cutting it to make room for more carbs or protein.