The Lean Gain Calculator estimates how many extra calories you should eat to gain weight at your chosen weekly rate, converts that to a daily calorie surplus (using 7,700 kcal ≈ 1 kg), estimates your BMR (Mifflin–St Jeor) and TDEE (activity × BMR), and then produces daily macronutrient targets (protein based on lean body mass, fat ≈25% of calories, and carbs to fill the remainder); it also offers a conservative estimate of how much of the weekly weight gain may be lean versus fat depending on training experience (beginners typically gain a higher fraction of lean mass), but remember these are approximations — use consistent resistance training, progressive overload, and regular body-composition checks and consult a coach or healthcare professional for an individualized program.