A standard medical definition of "normal" body weight is a BMI of about 18.5 - 24.9
A BMI under 18.5 indicates that the person is "underweight"
A BMI of 25.0 - 29.9 indicates that the individual is "overweight", but not obese
A BMI over 30 indicates obesity
A BMI over 40 indicates extreme obesity
Obesity is associated with increased risk for several serious disease processes. Extreme obesity is associated with a dramatically increased risk for many serious diseases. The web site weight.com is a good place to start learning more about those risks, as well as weight loss issues, etc.
Body mass index is calculated with this formula:
weight (in kilos) divided by height (in meters) squared
Therefore, if someone weighs 100 kilos and is 1.8 meters tall, their BMI is 100 divided by 1.8 X 1.8 = 100/3.24 = 30.9
To convert from pounds to kilos, take the weight in pounds and divide it by 2.2
To convert from inches to meters, take the height in inches times 0.0254