My eleven year old granddaughter has had significant weight gain (40 to 50 lbs) in the last year. She eats a healthy diet and does not overeat.?

What is BMI. More important than just her weight is her bmi, which also takes into account her sex and height based on her age. Quite honestly, a weight of 50# does not seem ecessive to me, but i would have to know her height as well. She is likely starting her pubertal growth spurt, and the weight gain may be totally appropriate. There are several good sites online to check bmi's for kids - check that first!