Consume fewer calories than you burn.
 

Consume fewer calories than you burn.

To figure out how many calories you burn a day, calculate your Resting Metabolic Rate (RMR)—the number of calories you burn daily doing routine activities, not including formal exercise—using this formula: RMR = bodyweight (in pounds) x 13. Next, determine how many calories you burn through exercise—a half-hour of moderate-intensity aerobic exercise burns around 350 calories in the average man, and a half-hour of lifting burns around 200. Add your RMR to the calories you burn in the gym, and keep your daily calorie consumption below that total.