What is Body Mass Index (BMI)?
Body Mass Index is a population-level screening number that compares a person's weight to their height. It was developed in the 1830s by the Belgian mathematician Adolphe Quetelet and was originally called the Quetelet Index. The World Health Organization adopted it in the 1990s as a low-cost way to flag whether an adult might be underweight, in a healthy range, overweight, or obese.
BMI is intentionally simple. It uses only two measurements, requires no equipment beyond a scale and a tape measure, and produces a single number that can be compared across large populations. That simplicity is also its main limitation — BMI cannot distinguish between fat mass and lean mass, and it does not account for where body fat is distributed.
The BMI formula
The metric formula is:
BMI = weight (kg) / height (m)²The imperial formula, used primarily in the United States, applies a conversion factor of 703:
BMI = (weight (lb) / height (in)²) × 703Both formulas produce the same number. A 70 kg adult who is 1.75 m tall has a BMI of 70 / 1.75² = 22.86, which sits in the middle of the healthy range. The result is unitless — the kilograms-per-square-meter unit cancels for interpretation.
Adult BMI categories (WHO)
| Category | BMI range | What it indicates |
|---|---|---|
| Underweight | < 18.5 | Possible nutritional deficiency or underlying illness |
| Healthy weight | 18.5 – 24.9 | Lowest statistical risk for weight-related disease |
| Overweight | 25.0 – 29.9 | Elevated risk; lifestyle review recommended |
| Obesity class I | 30.0 – 34.9 | High risk for type 2 diabetes and cardiovascular disease |
| Obesity class II | 35.0 – 39.9 | Very high risk; clinical intervention often warranted |
| Obesity class III | ≥ 40.0 | Extremely high risk; bariatric evaluation may be considered |
These cutoffs apply to most adults of European descent. The WHO and several Asia-Pacific health bodies use lower thresholds for people of Asian ancestry — overweight starts at 23.0 and obesity at 25.0 or 27.5 — because the same BMI corresponds to a higher percentage of body fat and a higher cardiometabolic risk in those populations.
Worked examples
- Adult, metric: 68 kg, 1.70 m →
68 / (1.70 × 1.70) = 23.5. Healthy weight. - Adult, imperial: 180 lb, 5 ft 10 in (70 in) →
(180 / 70²) × 703 = 25.8. Slightly overweight. - Edge case, athlete: A 95 kg rugby player at 1.80 m has a BMI of 29.3 — flagged as overweight despite having 10% body fat. BMI does not work well for high-muscle-mass individuals.
- Edge case, elderly: An 80-year-old at 55 kg and 1.65 m has a BMI of 20.2 — technically normal but may mask sarcopenia (loss of muscle mass) common in older adults.
When BMI is useful and when it isn't
BMI works best as a quick screen for sedentary or moderately active adults aged roughly 20 to 65. It is the basis for the United States National Heart, Lung, and Blood Institute's risk tables and is used in most clinical encounters to decide whether to pursue further measurements such as waist circumference, body fat percentage, or blood markers.
BMI is less reliable in the following situations:
- Highly muscular adults — bodybuilders, sprinters, and offensive linemen routinely score as overweight or obese.
- Pregnant or breastfeeding women — pregnancy-specific weight gain tables should be used instead.
- Children and teenagers — BMI for age and sex (BMI percentile) is used, not the adult categories.
- Older adults — a BMI of 25 to 27 may actually be associated with the lowest mortality in people over 70.
- People with very short or very tall stature — the formula's height-squared term distorts at the extremes.
Better complements to BMI
- Waist circumference: measured at the level of the navel; over 94 cm (men) or 80 cm (women) indicates elevated abdominal fat.
- Waist-to-height ratio: a simple rule of thumb — keep your waist under half your height.
- Body fat percentage: measured by skinfold calipers, bioelectrical impedance, or DEXA scan.
- Resting heart rate, blood pressure, and lipid panel for cardiometabolic risk.