What is Body Mass Index (BMI)?
Body Mass Index (BMI) is a simple numerical calculation that uses a person’s height and weight to estimate their body fatness. Developed by Adolphe Quetelet in the 19th century, it is now the most widely used screening tool for classifying weight categories in adults.
While BMI does not directly measure body fat, it correlates moderately well with direct measures of body fat obtained from skinfold thickness measurements, bioelectrical impedance, and DEXA scans.
How is BMI Calculated?
The standard formula for BMI calculation is:
BMI = weight (kg) ÷ height (m)²If you are using imperial measurements (pounds and inches), the formula requires a scaling factor of 703:
BMI = (weight in lbs × 703) ÷ height in inches²Our online BMI Calculator performs these calculations automatically using either unit system.
BMI Categories and Ranges
According to the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC), adults aged 20 and older are classified into the following categories based on their BMI:
- Underweight (BMI < 18.5): A BMI below 18.5 indicates that a person may be underweight, which can be associated with nutritional deficiencies or other underlying health issues.
- Normal Weight (18.5 – 24.9): A BMI within this range is considered healthy or optimal, indicating a low risk of weight-related health problems.
- Overweight (25.0 – 29.9): A BMI in this category indicates a person is carrying excess weight relative to their height.
- Obese (BMI ≥ 30.0): A BMI of 30 or higher is classified as obesity, which is further sub-divided into Class I (30.0–34.9), Class II (35.0–39.9), and Class III (≥ 40.0, sometimes referred to as severe obesity).
Limitations of BMI
While BMI is a useful population screening tool, it has several notable limitations for individuals:
- Does Not Differentiate Muscle and Fat: Muscle tissue is denser than fat. Athletes and bodybuilders with high muscle mass may have a BMI in the “overweight” or “obese” category despite having very low body fat.
- Does Not Account for Fat Distribution: BMI does not reflect where fat is stored on the body. Visceral fat (stored around internal organs in the abdomen) carries higher health risks than subcutaneous fat (stored under the skin).
- Varies by Age and Ethnicity: Natural body composition changes with age, and healthy ranges can vary across different ethnic groups. For example, some populations in Asia may have higher health risks at lower BMI values than Western populations.
Next Steps
To get a complete picture of your health, clinicians often use BMI in conjunction with other measurements such as waist circumference, body fat percentage, and blood pressure.