Body Mass Index (BMI) is an attempt to quantify the shape of the human body to determine whether an individual is a healthy weight for their height. The formula was devised in the mid-19th century and is simply the person’s mass (in kg) divided by the square of their height (in m). This number can then be used to determine whether or not an individual is a healthy average for their height, or under/overweight.
A BMI of <18.5 is said to be underweight, 18.5-25 is normal, 25-30 is overweight and a BMI >30 is classified as obese.
It’s important to note that BMI classification is not truly a reliable predictor of health or wellbeing, nor should you let yours dictate whether or not you feel the need to lose weight. Your weight loss goals should not involve moving down a BMI bracket and its important they are only based on the body you want and what you have determined to be healthy.
BMI is the best tool that we have for its purpose, and that purpose is doing statistics on large populations or making time-saving generalizations. There are a number of important factors that BMI does not take into account, the most obvious of which is that muscle weighs more than fat! According to BMI, George Clooney is thoroughly overweight and Arnold Schwarzenegger in his prime was positively obese. For fit, muscular individuals BMI is absolutely worthless as a metric of overall health.
Don’t pay any attention to your BMI classification. It’s not an important number and certainly not something that should concern a health-conscious individual. What matters is how you think you look, how you feel, and other valid markers of health like what your doctor has to say after a physical or how often and at what level you exercise. Being comfortable with these aspects of yourself, as well as maintaining a healthy diet and lifestyle, are the cornerstones of total wellbeing.