Approximately 300,000 deaths a year are attributed to obesity. Obesity is the second leading cause of preventable deaths in the United States.
Excessive weight can result in many serious, potentially life-threatening health problems, including:
BMI (Body Mass Index) tells you how your height / weight ratio compares to the height / weight ratio of average, healthy adults. It is the most widely used formula to identify weight problems. The formula is: body weight / height2.
The US/Imperial version is:
(Weight in lbs x 702) / Height in Inches Squared
The BMI formula can't tell if your weight is lean muscle mass or body fat. It's most accurate for people between 5' to 6'5" with average muscle mass. If you have a lot of lean muscle mass, it may tell you that you need to lose weight. If you're sporting a six pack and it gives you a big weight loss goal, don't worry about it.
Determining BMI categories for children is problematic. The calculator will give you a number, but Children should have lower BMI scores for adults, and unfortunately, it's a moving target. I don't recommend using BMI or the "ideal weight" formulas for children. However, if it tells you they should lose weight, it is almost certainly right about that. It will tell you that most children are under-weight, even if they're not.