Understanding Body Mass Index (BMI)
Body Mass Index (BMI) is one of the most widely utilized screening tools in clinical medicine, epidemiology, and public health. Developed in the 19th century by Belgian polymath Adolphe Quetelet, BMI provides a straightforward numerical estimate of a person's relative body mass normalized for their height.
The Mathematical Formula for BMI
The calculation of BMI requires only two fundamental physical measurements: body weight and height. Depending on your preferred system of measurement, use either the metric or imperial formula below:
Metric Formula: BMI = Weight (kg) / [Height (m)]²
Imperial Formula: BMI = [Weight (lbs) × 703] / [Height (inches)]²
World Health Organization (WHO) Standard Classifications
For adults aged 20 years and older, the World Health Organization defines the following universal cutoff ranges:
- Underweight: BMI below 18.5 kg/m²
- Normal / Healthy Weight: BMI between 18.5 and 24.9 kg/m²
- Overweight: BMI between 25.0 and 29.9 kg/m²
- Class I Obesity: BMI between 30.0 and 34.9 kg/m²
- Class II Obesity: BMI between 35.0 and 39.9 kg/m²
- Class III Obesity (Severe): BMI 40.0 kg/m² or higher
Why BMI Remains Relevant
Despite modern advances in body composition imaging (such as DEXA scans and bioelectrical impedance), BMI remains invaluable in clinical research because it is non-invasive, cost-effective, and strongly correlated with population-level morbidity and mortality risks.
To calculate your personal BMI score with instant WHO classification charts, visit our interactive BMI Calculator.