Body Mass Index (BMI) is a value used to get a general sense of a person’s overall mass and is calculated using a person’s height and weight.[3] It is more often used than weight alone to determine if an individual is underweight, normal weight, overweight, or obese. The following two equations can used to calculate BMI depending on the units used for height (meters vs. inches) and weight (kilograms vs. pounds):[3]

BMI = Weight (kg) / Height2 (m2)

OR

BMI = [ Weight (lbs) / Height2 (in2) ] x 703

Though BMI is often times used to help assess for excess weight, it is by no means a perfect representation of a person’s body fat percentage.[3] For example, an individual can have a higher than normal BMI but have a normal body fat percentage if they have higher than average muscle mass because excess muscle contributes to a higher weight. The following table shows how different ranges of BMIs are often categorized into underweight, normal weight, overweight, and obese:[3]

Classification of Overweight and Obesity by Body Mass Index (BMI)
Obesity Class BMI
Underweight {\displaystyle <} 18.5
Normal Weight 18.5 – 24.9
Overweight 25.0 – 29.9
Obesity I 30.0 – 34.9
Obesity II 35.0 – 39.9
Extreme Obesity III {\displaystyle \geq }40.0

Since BMI is not a perfect representation of a person’s body fat percentage, other measurements like waist circumference are often used to better assess for unhealthy excess weight as it pertains to body fat. Despite not being a perfect representation of healthy and unhealthy weight, BMI is very important value because it helps health professionals identify people who are at higher risk of developing illnesses like diabetes, hypertension, dyslipidemia (high cholesterol), liver disease, and some cancers. In general, as BMI increases so too does a person’s risk of developing those previously stated illnesses. Additionally, regularly calculating a person’s BMI can also be used to help track changes in a person’s body mass over time.