What Is BMI and How Is It Calculated?

Body Mass Index (BMI) is a simple ratio of weight to height first proposed by Belgian statistician Adolphe Quetelet in the 19th century. Today it is used by the World Health Organization, the NHS in the United Kingdom, and the U.S. Centers for Disease Control and Prevention as a quick screening tool for whether an adult's weight is in a generally healthy range.

The BMI formula

BMI is calculated by dividing weight in kilograms by height in metres squared. In imperial units the formula multiplies the result by a conversion constant: weight in pounds, divided by height in inches squared, multiplied by 703.

WHO BMI categories for adults

• Underweight: below 18.5
• Healthy weight: 18.5 to 24.9
• Overweight: 25.0 to 29.9
• Obesity class I: 30.0 to 34.9
• Obesity class II: 35.0 to 39.9
• Obesity class III: 40.0 and above

What BMI is good at

BMI works well as a population-level screening tool. It is cheap to measure, requires no special equipment, and correlates broadly with health outcomes across large groups.

Where BMI falls short

BMI doesn't distinguish between fat and muscle. A heavily-trained rugby player can register as 'overweight' or 'obese' on BMI alone. It also doesn't account for fat distribution, ethnicity-specific risk thresholds, age-related muscle loss, or pregnancy.

For a fuller picture, pair BMI with a waist-to-hip ratio measurement and, if available, a body fat percentage estimate.