What is BMI?
Body Mass Index (BMI) is a simple number calculated from your height and weight. It provides a rough estimate of whether you’re carrying a healthy amount of body fat relative to your size.
BMI was developed by Adolphe Quetelet in the 19th century and later adopted by the World Health Organization (WHO) as a population-level screening tool.
How BMI is Calculated
BMI = Weight (kg) / Height (m)²
Example: Weight 70 kg, Height 1.75 m
BMI = 70 / (1.75)²
= 70 / 3.0625
= 22.9The WHO BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | ✅ Normal / Healthy weight |
| 25.0 – 29.9 | ⚠️ Overweight |
| 30.0 – 34.9 | Obese (Class I) |
| 35.0 – 39.9 | Obese (Class II) |
| 40.0 and above | Severely obese (Class III) |
What Each Category Means in Practice
Underweight (< 18.5)
Can indicate malnutrition, an eating disorder, or an underlying illness. Associated with weakened immunity, bone density loss, and in severe cases, organ failure. Worth discussing with a doctor.
Normal weight (18.5 – 24.9)
Associated with the lowest risk of weight-related health conditions in population studies. This doesn’t guarantee health — diet, fitness, and lifestyle matter enormously.
Overweight (25 – 29.9)
Slightly elevated risk of heart disease, type 2 diabetes, and high blood pressure. For many people in this range, small lifestyle changes — more movement, less processed food — have a meaningful impact.
Obese Class I (30 – 34.9)
Substantially elevated health risks. Weight loss of even 5–10% of body weight is associated with meaningful improvements in blood pressure, cholesterol, and blood sugar.
Obese Class II & III (35+)
High to very high risk across multiple conditions. Medical guidance is strongly recommended.
Where BMI Falls Short
BMI is a blunt instrument. It measures weight relative to height, not body composition.
- Athletes and muscular people — muscle is denser than fat. A fit athlete may have a BMI of 27 (classified as overweight) but very low body fat.
- Older adults — people naturally lose muscle as they age. An older adult may have a “normal” BMI while carrying proportionally more fat than is healthy.
- Different ethnic groups — research suggests people of South Asian and East Asian backgrounds face health risks at lower BMI thresholds. Some authorities recommend a lower cutoff of 23 for South Asians.
- Doesn’t reflect fat distribution — abdominal fat carries more health risk than fat stored in the hips and thighs. Waist circumference is a better predictor of metabolic risk than BMI alone.
BMI as a Starting Point, Not a Verdict
Think of BMI as a quick triage tool — not a diagnosis. It tells you roughly where you are on a population scale and whether your weight warrants a closer look.
- Use waist circumference too (above 80 cm for women, 94 cm for men increases health risk)
- Check body fat percentage via DEXA scan, hydrostatic weighing, or skinfold calipers
- Consider fitness markers: resting heart rate, blood pressure, blood glucose
- Don't use BMI as a final diagnosis — it's a screening tool, not a verdict
- Don't ignore ethnicity-specific thresholds — South Asians may face risk at BMI 23+
- Don't apply adult BMI ranges to children — use age- and sex-specific percentile charts
Calculate Your BMI Now
Use our free BMI Calculator to get your BMI instantly in both metric and imperial units, with your WHO category and a personalized health note.