Calorie Needs to gain weight
Once you know the number of calories you need to maintain your weight (using our BMR calculator in conjunction with our Harris Benedict equation, you can easily calculate the number of calories you need in order to gain weight.
If you want to gain body weight, you need to consume more calories than you burn. One pound of body weight is roughly equivalent to 3,500 calories, so eating an extra 500 calories per day will cause you to gain one pound a week.
For optimum health, if you increase your calories to gain weight then (health permitting) gradually increase your level of physical exercise in order to maintain or increase your lean body mass. The benefits of exercise on physical and mental health are well documented and shouldn't be ignored.