What is BMI? BMI is Body mass index. A measure that relates body weight to height. BMI is sometimes used to measure total body fat and whether a person is at a healthy weight. Excess body fat is linked to an increased risk of some diseases including heart disease and some cancers. A BMI under 18.5 is described as underweight. between 18.5 and 24.9 is described as a 'healthy range'. between 25 and 29.9 – This is described as overweight. between 30 and 39.9 – This is described as obesity.