BMI (Body Mass Index) is a simple formula that uses height and weight to estimate whether a person falls into an underweight, normal weight, overweight, or obese category. While it is widely used in medicine and public health as a screening tool, BMI has significant limitations — it does not distinguish muscle from fat, or account for age, gender, or ethnicity differences in body composition. This page covers both the metric and imperial BMI formulas, the WHO category definitions, and the key caveats.