The body mass index is basically just a ratio of weight to height, taking into account the fact that taller people have more body mass than shorter people, and, therefore, they are going to weigh more. Athletes and muscular people will have a higher body mass index because bone and muscle are more dense than fat. Unfortunately, for most adults, almost all weight gain is from fat and not muscle or bone mass.
Your main goal is to maintain a body mass index (BMI) below 25. Several studies have shown that people who have a BMI above 25 are much more likely to die early, probably from heart disease and cancer. When your body mass index gets above 30, these chances increase dramatically.
Being overweight is considered having a BMI above 25 and below 30. If you have a body mass index above 30, you are then considered obese.
There is no special event or dramatic occurrence when you cross the line above the BMI of 25 or 30. It is just simply a reference point to which the increased risk of obtaining weight-related disease occurs when you are in those ranges.
It’s pretty simple to calculate your body mass index yourself. All you need to do is simply divide your weight in pounds by your height in inches. Then multiply that answer by 703. The National Heart, Lung and Blood Institute also a BMI calculator on their website at http://www.nhlbisupport.com/bmi/ .
Sources:
http://www.bmi-calculator.net/
http://www.cdc.gov/healthyweight/assessing/bmi/
Abdominal Obesity and the Risk of All-Cause, Cardiovascular, and Cancer Mortality. Sixteen Years of Follow-Up in US Women. Circulation. 2008