The amount of weight a person can gain in one week depends on factors, such as the individual's metabolism and the amount of calories consumed, according to SFGate. Weight gain is a process that relies on the body taking in and storing more calories than it burns.
A person burns 2,000 calories per day on average, depending on age, gender, physical activity levels and body size, states SFGate. A pound of fat is equal to 3,500 calories. In order to gain 1 pound, a person must consume and store an additional 3,500 calories more than he normally eats to maintain his current weight. Depending on the rate at which the person overeats, over time excess calories accumulate and are stored as body fat, and body weight increases.