A person of 150 pounds swimming at normal intensity could swim a single kilometer in 20 minutes. While swimming at that intensity over that length of time, the person would burn 250 calories.
Swimming is a very effective method of burning calories and exercise. Swimming is low impact and does not cause the strain that other higher impact exercises cause. The faster one swims, the more calories they burn. If one extends their swim from 20 minutes to one hour, they could burn as many as 511 calories. The speed at which a person might swim a kilometer would depend on his or her weight and swimming ability.