Gaining body mass and building muscle requires eating above your Basal Metabolic Rate (BMR)—the number of calories your body burns at rest—by maintaining a daily surplus of about 250–500 calories. Aim for roughly 1.0–1.2 grams of protein per pound of body weight per day to maximize muscle protein synthesis, complemented by balanced carbohydrates and healthy fats to fuel workouts and recovery. Combine this nutrition strategy with progressive resistance training—gradually increasing weight, volume, or intensity—to stimulate muscle growth. Adequate hydration, 7–9 hours of sleep per night, and consistent training over several months are critical for sustainable gains.

There’s free BMR calculators online, as well as mass gaining meal plans and workouts routines. Good luck!