I'm 16, 5' 5" and weigh just over 7 and a half stone.

I know this means I'm underweight and its important that I put some back on

Does anyone know how many calories I should be getting a day until I get back to a healthy weight as I don't want to do it by binge eating.

Are you sure you are underweight? If you are fit and healthy, with plenty of energy and don't have problems with bones sticking into things, you may just be naturally light weight, not underweight.

If you are really underweight, then gradually increase your overall calories in several small meals by increasing the amount of good quality fresh food you eat. Plan to eat things like eggs, salmon, steak, fruit, vegetables, nuts, seeds, olive oil, dairy and whole grains. Not sugar or refined food.

If you do vigorous exercise, then try having a whey and dextrose shake at the end.