Your body mass index approximates your level of body fat. The BMI factors your height and weight into its calculation. The index is the same for adult men and women. While it is only an estimation, medical authorities state the BMI generally does a good job predicting fat levels. More complex ways to figure body fat exist, but physicians and other health authorities rely on the BMI because it's relatively accurate and cheap. The calculation isn't perfect or without critics, but it remains the most common measure of obesity.
Background
In 1832 a Belgian mathematician developed the BMI, but the formula didn't gain traction as a means to determine obesity until the late 20th century. The index compares your height and weight to the population at-large. Other methods such as calipers, underwater weighing and radiology can also determine obesity, but the BMI is comparatively cheaper and effortless.
Calculation
To get your BMI, take your weight in pounds, divide by your height in inches squared and then multiply by 703. Health authorities classify a BMI below 18.5 as underweight, a BMI between 18.5 and 24.9 as normal and a BMI between 25 and 29.9 as overweight. If your BMI is 30 or more, you are classified as obese.
Interpretation
While you can figure out your own BMI, your physician can best interpret what your score really means. As a general rule, if you are overweight or obese you have a higher risk of getting a host of ailments such as diabetes, heart disease and high blood pressure. Still, a high BMI doesn't necessarily mean you're unhealthy. Even the National Heart, Lung and Blood Institute notes its imperfection, which is why you should consult your physician to best understand your score.
Shortcomings and Criticism
Your BMI is just one way to measure your health. If you're athletic, the BMI may overestimate your fat levels as muscle weights more than fat. Lean muscle mass often declines with age, so the BMI may underestimate fat levels in older adults. Men and women use the same formula to determine their BMI. Men tend to have less fat than women, however, so a woman with the same BMI as a man probably has more fat.
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