Body Mass Index (BMI) is a measurement that helps determine if someone is underweight, overweight, or at a healthy weight for their height. The National Institute of Health (NIH) uses BMI as the deciding factor for these categories:
Underweight: Less than 18.5
Healthy weight: 18.5–24.9
Overweight: 25–29.9
Obese: 30–39.9

A person who is 5 ft (60 in) and weighs 60 kg has a BMI of 22.9, calculated by dividing 60 kg by 1.6 m² (1.6 x 1.6), which is the square of 5 ft 3 in (1.6 m).

BMI can be influenced by age, gender, and ethnicity, so it should be used as a guide only. Other factors, such as waist size, can also be used to assess health.