Set a calorie surplus for muscle gain.
A kilogram of body mass carries roughly 7,700 calories of stored energy, so a target rate of gain converts directly into a daily surplus. Keeping the surplus modest — a quarter of a kilogram a week for most people — is what biases the gain toward muscle rather than fat. Protein is set per kilogram of body weight and rises when you train with resistance at least three times a week, because the surplus only builds muscle if training and protein support it.
Lean gain surplus
Daily surplus = target weekly gain x 7700 / 7; daily target = maintenance calories + daily surplus; protein = body weight x 1.8 g when training 3+ days
Nutrition targets here are general estimates, not medical or dietetic advice. Individual needs vary with health conditions, medication and body composition — consult a qualified professional before changing your diet.
For most people 250-500 calories a day. Larger surpluses add weight faster but a greater share of it is fat, which then has to be dieted off.
Extra calories only become muscle when there is a training stimulus and enough protein to build with. Without resistance training, a surplus mostly adds fat.
Raise the surplus by 100-150 calories and reassess after two weeks. Maintenance estimates are approximate and daily weight fluctuates with water and food volume.