Body Mass Index, commonly known as BMI, is a widely used screening measurement that helps assess whether a person's weight is in a healthy range relative to their height. Developed in the 1830s by Belgian mathematician Adolphe Quetelet, BMI has become the standard tool used by healthcare providers, public health organizations, and researchers worldwide to categorize weight status and identify potential health risks associated with being underweight, overweight, or obese.
While BMI is not a direct measure of body fat percentage, it provides a quick, non-invasive, and cost-free way to identify individuals who may be at risk for weight-related health conditions. The World Health Organization, the Centers for Disease Control and Prevention, and the National Institutes of Health all use BMI as a primary screening tool in clinical and population-level health assessments. Understanding your BMI is therefore an important first step in taking ownership of your long-term health.
How BMI Is Calculated
The BMI formula is straightforward: divide your weight in kilograms by the square of your height in meters. For those using imperial measurements, the formula is: BMI = (weight in pounds × 703) ÷ (height in inches)². The resulting number falls into one of four WHO-defined categories: below 18.5 is Underweight, 18.5 to 24.9 is Normal weight, 25.0 to 29.9 is Overweight, and 30.0 or above is Obese.
For a concrete example: a person who is 5 feet 9 inches tall (175 cm) and weighs 175 pounds (79.5 kg) has a BMI of approximately 25.8, placing them in the Overweight category. The same person at 160 pounds (72.6 kg) would have a BMI of 23.6, comfortably in the Normal range.
The healthy weight range varies significantly by height. A person who is 5 feet 4 inches tall has a healthy weight range of approximately 108 to 145 pounds. A person who is 6 feet tall has a healthy range of approximately 136 to 183 pounds. These ranges are why BMI must always be interpreted in the context of height, not just absolute weight.
BMI Categories and Associated Health Risks
Each BMI category is associated with different health risk profiles, though it is critical to understand that BMI is a screening tool, not a diagnostic measure. Being in the Overweight or Obese category increases statistical risk for type 2 diabetes, cardiovascular disease, hypertension, certain cancers, sleep apnea, osteoarthritis, and all-cause mortality.
The Obese category is further subdivided by many healthcare providers into Class 1 (BMI 30.0–34.9), Class 2 (BMI 35.0–39.9), and Class 3 (BMI 40.0 and above, sometimes called extreme or morbid obesity). Higher obesity classes are associated with significantly elevated health risks and may qualify individuals for medical interventions including bariatric surgery.
Being Underweight (below 18.5) carries its own distinct health risks including malnutrition, weakened immune function, bone density loss, anemia, and fertility problems. Underweight status may indicate underlying conditions such as eating disorders, hyperthyroidism, or malabsorption disorders that warrant medical evaluation.
Limitations of BMI: What It Does Not Measure
BMI's primary limitation is that it does not distinguish between fat mass and lean mass. A muscular athlete might have a BMI in the Overweight range despite having very low body fat, while a sedentary person with little muscle mass might have a Normal BMI despite having high body fat — a condition sometimes called "skinny fat" or normal-weight obesity.
Age, sex, ethnicity, and bone structure also affect how BMI should be interpreted. Women naturally carry more body fat than men at the same BMI. Older adults typically have more body fat than younger adults at the same BMI. Research suggests that Asian populations may face elevated health risks at lower BMI thresholds than the standard WHO categories suggest — some Asian health organizations use a cut-off of 23.0 for Overweight rather than 25.0.
For a more complete picture of health and body composition, healthcare providers may use waist circumference, waist-to-hip ratio, body fat percentage measurements (via DEXA scan, hydrostatic weighing, or bioelectrical impedance), and blood markers alongside BMI.
BMI is a useful starting point for understanding your weight-related health status, but it is only one piece of a larger health picture. Use this calculator as a guide, but work with your healthcare provider to interpret your results in the context of your overall health, lifestyle, fitness level, and family history. The most important insight BMI can provide is not a judgment, but a prompt for a conversation with your doctor about what steps, if any, might support your long-term wellbeing.