Consuming 3,500 more calories will add a pound of body fat to a person, according to the Mayo Clinic. For years, that's been used as the basic truth behind gaining a pound.
However, other sources base gains on such tools as formulas. The calorie intake number then isn't quite fixed, but rather it may take into account a person's weight. Muscle & Fitness, a bodybuilding magazine, tells its readers to multiply their weight in pounds by 20. The result is the number of extra calories they'd need to produce gains. By that measure, if someone's 200 pounds, 3,500 would fall short; 4,000 would be required.