What Is BMI?
BMI (Body Mass Index) is a numerical value calculated from a person's weight and height. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s as a population-level statistical tool — not as a clinical diagnostic for individuals. The World Health Organization adopted it as a convenient obesity screening tool in the 1990s.
BMI does not directly measure body fat. It uses the ratio of weight to height squared as a proxy for body composition. It is a screening tool, not a diagnostic test.
The BMI Formula
Metric (kg and cm)
BMI = weight (kg) / height (m)²
Note: height must be in metres, not centimetres. Divide cm by 100 first.
Imperial (lbs and inches)
BMI = 703 × weight (lbs) / height (in)²
The factor 703 converts lbs/in² to the same scale as kg/m².
Worked Examples
Example 1 — Metric: 75 kg, 175 cm
Height in metres: 175 cm ÷ 100 = 1.75 m
BMI = 75 / (1.75)² = 75 / 3.0625 = 24.5
Category: Normal weight
Example 2 — Imperial: 165 lbs, 5'9" (69 inches)
BMI = 703 × 165 / (69)² = 115,995 / 4,761 = 24.4
Category: Normal weight
BMI Categories (WHO Standard)
For Asian populations, some health authorities use lower thresholds: overweight at BMI ≥ 23, obese at BMI ≥ 27.5, due to higher metabolic risk at lower BMI values.
Limitations of BMI
Does not measure body fat directly
BMI cannot distinguish between fat mass and lean mass. A bodybuilder with very little fat may be classified as obese by BMI, while a sedentary person with low muscle mass ("skinny fat") may be in the normal range despite unhealthy body composition.
Age and sex differences
Older adults typically have more fat at the same BMI as younger adults. Women naturally carry more body fat than men at the same BMI. These differences are not captured by a single universal scale.
Ethnicity differences
Studies show that Asian, South Asian, and some other ethnic groups have higher health risks at lower BMI values compared to European populations, prompting WHO to recommend adjusted thresholds.
Does not capture fat distribution
Visceral fat (around internal organs) is far more dangerous than subcutaneous fat. Two people with the same BMI can have very different visceral fat levels. Waist circumference is often a better predictor of metabolic risk.
What Should You Use Instead of BMI?
BMI is a starting point, not the whole picture. Clinicians often use it alongside:
- •Waist circumference — risk increases above 40 inches (102 cm) in men and 35 inches (88 cm) in women
- •Waist-to-height ratio — a ratio above 0.5 indicates increased cardiometabolic risk regardless of BMI
- •Body fat percentage — measured by DEXA scan, hydrostatic weighing, or the Navy circumference method
- •Blood markers — fasting glucose, HbA1c, lipid panel, blood pressure are more direct metabolic risk indicators
The Bottom Line
BMI is a quick, free, population-level screening tool that requires nothing more than height and weight. Despite its limitations, a BMI above 30 is still associated with significantly higher risks of type 2 diabetes, cardiovascular disease, and certain cancers. It is best used as a starting conversation with your doctor — not as a definitive health verdict. For a more complete picture, pair your BMI with waist circumference and body fat percentage measurements.