Everything you need to know about BMI — from how it's calculated to what your number actually means, why Asian thresholds differ, and what BMI can't tell you.
Body Mass Index (BMI) is a simple screening measure that uses your height and weight to estimate whether you're underweight, normal weight, overweight, or obese. It was developed by Belgian mathematician Adolphe Quetelet in the 1830s.
BMI = Weight (kg) ÷ Height (m)²
For example, a person weighing 70 kg and 1.75 m tall: BMI = 70 ÷ (1.75 × 1.75) = 70 ÷ 3.0625 = 22.9
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 16.0 | Severe underweight | High (malnutrition, immune weakness) |
| 16.0 – 17.0 | Moderate underweight | Moderate |
| 17.0 – 18.5 | Mild underweight | Low to moderate |
| 18.5 – 24.9 | Normal weight | Low |
| 25.0 – 29.9 | Overweight | Increased |
| 30.0 – 34.9 | Obese Class I | High |
| 35.0 – 39.9 | Obese Class II | Very high |
| 40.0+ | Obese Class III | Extremely high |
The "normal" range of 18.5–24.9 has the lowest statistical risk for type 2 diabetes, cardiovascular disease, and all-cause mortality in large population studies.
Research shows that Asian populations develop obesity-related diseases (diabetes, heart disease) at lower BMI values compared to Western populations. The WHO and Indian health authorities use revised thresholds for Asians:
| Category | WHO (International) | Asian / Indian |
|---|---|---|
| Underweight | Below 18.5 | Below 18.5 |
| Normal | 18.5 – 24.9 | 18.5 – 22.9 |
| Overweight | 25.0 – 29.9 | 23.0 – 24.9 |
| Obese | 30.0+ | 25.0+ |
For Indian adults: Aim for a BMI of 18.5–22.9 and waist circumference below 90 cm (men) or 80 cm (women).
BMI is a useful starting point, not the final word. Always combine it with other measurements and health markers.
| Metric | What It Measures | Healthy Range |
|---|---|---|
| Waist circumference | Visceral fat around organs | Men < 90 cm, Women < 80 cm (Asian) |
| Waist-to-hip ratio | Fat distribution pattern | Men < 0.9, Women < 0.85 |
| Body fat percentage | Actual fat vs lean mass | Men 10–20%, Women 18–28% |
| Waist-to-height ratio | Proportional belly fat | Below 0.5 for both sexes |
Many researchers consider waist-to-height ratio the single best predictor of heart disease risk. The rule is simple: your waist should be less than half your height. If you're 170 cm tall, your waist should be under 85 cm.
BMI for children works differently from adults. Because body fat changes with age and differs between boys and girls, children's BMI is interpreted using age- and sex-specific percentiles.
| Percentile Range | Category |
|---|---|
| Below 5th percentile | Underweight |
| 5th – 85th percentile | Healthy weight |
| 85th – 95th percentile | Overweight |
| 95th percentile and above | Obese |
A 10-year-old boy with a BMI of 21 is at the 90th percentile (overweight), while an adult with BMI 21 is perfectly normal. Always use age-specific growth charts for children.
No. Athletes with significant muscle mass often have "overweight" or "obese" BMI despite having very low body fat. Use body fat percentage or waist-to-height ratio instead.
Population studies show a J-shaped curve: very low (<18.5) and very high (>35) BMIs are associated with higher mortality. The lowest risk is in the 20–25 range for most populations, and 18.5–23 for Asians.
Once a month is sufficient. Weight fluctuates daily due to water retention, meals, and exercise. Track trends over weeks, not individual readings.
Not necessarily. BMI 25–27 with good fitness, normal waist circumference, and healthy blood markers carries minimal extra risk. Context matters more than the number alone.