How many calories should you eat to gain 1 kilo?

To gain one kilogram of body mass, including fat and muscle, you need to consume approximately 7,700 calories above your regular maintenance calories. This means eating 500-1,000 calories more per day for 7-8 days, or 250-500 calories more per day for 15-16 days. Keep in mind that while gaining muscle is the goal, it's important to focus on a healthy and sustainable weight gain process, including regular exercise, a balanced diet, and proper rest.