What is BMI, and should you trust it?
Body Mass Index is one of the most widely used health screening tools in the world — and also one of the most misunderstood. Here's everything you need to know.
The formula.
BMI was invented in the 1830s by Belgian mathematician Adolphe Quetelet — not a doctor. It was designed as a population statistics tool, not an individual health assessment.
Metric: BMI = weight(kg) ÷ height(m)²
Imperial: BMI = [weight(lbs) ÷ height(in)²] × 703
Standard categories.
| Category | BMI Range | Risk level |
|---|---|---|
| Underweight | < 18.5 | Increased |
| Normal weight | 18.5 – 24.9 | Low |
| Overweight | 25 – 29.9 | Moderate |
| Obese (Class I) | 30 – 34.9 | High |
| Obese (Class II) | 35 – 39.9 | Very High |
| Obese (Class III) | ≥ 40 | Extremely High |
The three standards we support.
CDC Standard
The US Centers for Disease Control uses standard adult thresholds (above) for all adults. For children and teens aged 2–19, it uses percentile charts comparing to peers of the same age and sex — because healthy BMI ranges shift significantly during development.
NHS Standard (Ethnicity-Adjusted)
The UK's National Health Service recognises that South Asian and Chinese individuals carry significantly higher cardiometabolic risk at lower BMIs. For these groups, the healthy range is 18.5–22.9, overweight begins at 23, and obese begins at 27.5. This reflects higher visceral fat storage at equivalent BMIs.
WebMD Body Frame Standard
WebMD's approach estimates your body frame size from wrist circumference relative to height. Large-framed individuals naturally carry more bone and muscle mass, so they can be healthy at higher BMI values. Small-framed individuals may be at risk at lower BMIs. This provides a more personalised threshold.
Why BMI has real limitations.
- Ignores body composition. A muscular athlete can have a "obese" BMI while having 12% body fat. BMI cannot distinguish fat from muscle.
- Ignores fat distribution. Visceral fat (around organs) is far more dangerous than subcutaneous fat (under the skin). BMI measures neither.
- Gender blind. Women naturally carry more body fat than men at the same BMI. The formula was calibrated on male bodies.
- Age insensitive. Older adults tend to have more body fat at the same BMI. The healthy range shifts with age.
- Ethnic variation. As reflected in the NHS standard — different populations carry risk differently.
Bottom line: BMI is a useful screening tool, not a diagnostic one. It's one data point among many. SuperBMI combines it with sleep, hydration, exercise, and mood for a more complete picture.
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Medical disclaimer: The information on this website is for educational and informational purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition.