Body mass index, or BMI, is a measure that compares your weight to your height to assess if your weight is healthy. It is calculated by dividing weight in kilograms by height in metres squared. A BMI between 18.5 and 24.9 is considered healthy, 25 to 29.9 is overweight, and 30 or more is obese. You can use your height and weight to calculate your body mass index (BMI) and determine your weight category.