A minimum of 7000 calories are required to increase your body weight by 1 kg. If you want to gain weight, you need to eat at least 500 to 1000 calories more than you normally would eat in a day. At this rate, you would have gained 1 kg by the end of 1 or 2 weeks, depending on your intake. Also, keep in mind that the calories you burn also impact the rate of weight gain. The more calories you burn, the slower you will gain weight. Read more