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BMI (Body Mass Index) is a value derived from the mass (weight) and height of a person. The BMI is defined as the body mass divided by the square of the body height, and is expressed in units of kg/m². The formula is: BMI = weight (kg) / [height (m)]².

BMI is a useful screening tool but does have limitations. It doesn’t directly measure body fat or account for factors like muscle mass, bone density, or overall body composition. Athletes with high muscle mass may have a high BMI but low body fat. For most people, however, BMI correlates with body fat and related health risks.

Underweight may indicate malnutrition, osteoporosis, anemia, or other health issues. Normal weight is associated with the lowest health risks. Overweight increases the risk of cardiovascular disease, diabetes, and certain cancers. Obesity significantly raises the risk of these conditions plus others like sleep apnea, fatty liver disease, and joint problems.

For most adults, checking your BMI once or twice a year is sufficient, unless you’re actively trying to lose or gain weight. If you’re making lifestyle changes, you might check it monthly to track progress. Remember that BMI is just one indicator of health, and it should be considered alongside other factors like waist circumference, blood pressure, and blood tests.

BMI can be used for children and teens, but the interpretation is different. For those under 20, BMI is age and sex-specific and is often expressed as a BMI percentile. This compares the child’s BMI with that of other children of the same age and sex. A BMI above the 85th percentile is considered overweight, and above the 95th percentile is considered obese.