This article is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional for personalised health guidance.
BMI stands for Body Mass Index — a numerical value derived from a person's weight and height that is used as a screening tool for body weight categories. It was developed by Belgian mathematician Adolphe Quetelet in 1832, who called it the "Quetelet Index." The term "Body Mass Index" was coined by Ancel Keys in 1972, who studied it as a population-level tool for examining relationships between body weight and health outcomes.
BMI is calculated with a simple formula and was designed to be applied across large populations, not to assess individual health. This distinction — population screening tool versus individual diagnostic — is the most important thing to understand about BMI's limitations and appropriate use.
Metric: BMI = weight (kg) ÷ height (m)²
Imperial: BMI = (weight (lbs) × 703) ÷ height (inches)²
Example (metric): A person who weighs 70 kg and is 1.75 m tall:
BMI = 70 ÷ (1.75)² = 70 ÷ 3.0625 = 22.9
Example (imperial): A person who weighs 154 lbs and is 5'9" (69 inches) tall:
BMI = (154 × 703) ÷ (69)² = 108,262 ÷ 4,761 = 22.7
Use Criply's BMI calculator to get an instant result in either unit system.
The World Health Organization defines four BMI categories for adults:
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
Within the obese category, some classifications further subdivide into Class I (30–34.9), Class II (35–39.9), and Class III (40+), though the basic four-category system is the most widely used in everyday contexts.
BMI measures weight relative to height, not body composition. Muscle is significantly denser than fat — a person who is very muscular will have a higher weight for their height, and therefore a higher BMI, without having excess body fat. Many professional athletes register as "overweight" or even "obese" by BMI despite having low body fat percentages. A 180 lb, 5'10" marathon runner and a 180 lb, 5'10" sedentary person have identical BMIs but very different health profiles.
Where fat is stored on the body matters as much as how much there is. Visceral fat — the fat stored around internal organs in the abdominal area — is more strongly associated with metabolic disease, cardiovascular risk, and type 2 diabetes than subcutaneous fat stored under the skin elsewhere on the body. BMI does not measure this distinction at all. Waist circumference and waist-to-hip ratio are better proxies for abdominal fat than BMI.
The standard BMI thresholds were developed from studies on predominantly white European populations. Research has shown that people of South, East, and Southeast Asian descent face higher cardiometabolic risk at BMI values that fall in the "normal" Western range. Many health organisations now recommend lower overweight thresholds (23 rather than 25) for Asian populations. Sex differences also exist: at the same BMI, women generally have higher body fat percentages than men.
In adults over 65, a slightly higher BMI may be associated with better outcomes — this is sometimes called the "obesity paradox." In children and adolescents, BMI is calculated but must be interpreted against age and sex-specific growth charts (BMI percentiles) rather than the adult category thresholds.
Where BMI is useful:
Where BMI falls short:
For a more complete picture, healthcare professionals use:
BMI remains a standard clinical screening metric because it is cheap, fast, and reproducible. The right interpretation is: BMI is one piece of information, not a health verdict. Use it as a starting point for a conversation with a healthcare professional, not as a conclusion.
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