A person must consume 3,500 calories to gain 1 pound, according to Mayo Clinic; however, the body uses calories even while sleeping or sitting. Technically, a person must consume 3,500 calories above and beyond what his body uses to maintain itself to gain 1 pound.
A person who eats more calories than he burns gains weight, notes Mayo Clinic. A person who consumes 3,500 calories worth of food over the course of a day but burns 2,500 calories does not gain a pound of weight. A person who reduces his caloric intake by 500 calories a day over the course of a week loses 1 pound.