What is BMI?
BMI (Body Mass Index) is a simple numerical value calculated from your height and weight. It is widely used as a screening tool to classify whether a person is underweight, normal weight, overweight, or obese. BMI was developed by Belgian mathematician Adolphe Quetelet in the 1830s and is now used by the World Health Organisation (WHO) as a standard population health metric.
While BMI is not a direct measure of body fat percentage, it provides a quick, cost-free indicator that correlates well with more direct measures of body fat for most adults.
BMI Formula — Metric and Imperial
Example (Metric): A person who is 170 cm tall and weighs 70 kg:
Height in metres = 1.70 m → BMI = 70 ÷ (1.70)² = 70 ÷ 2.89 = 24.2
Example (Imperial): A person who is 67 inches tall and weighs 154 lbs:
BMI = (154 ÷ 67²) × 703 = (154 ÷ 4489) × 703 = 24.1
BMI Categories — WHO Standard Ranges
| BMI Range | Category | Health Risk |
|---|---|---|
| Below 18.5 | Underweight | Increased risk of malnutrition, osteoporosis |
| 18.5 – 24.9 | Normal Weight | Lowest health risk |
| 25.0 – 29.9 | Overweight | Moderate risk of heart disease, diabetes |
| 30.0 – 34.9 | Obese Class I | High risk |
| 35.0 – 39.9 | Obese Class II | Very high risk |
| 40 and above | Obese Class III | Extremely high risk |
💡 Asian populations: WHO recommends lower BMI thresholds for Asian adults. Overweight starts at BMI 23 and obesity at BMI 27.5 for South Asian and East Asian individuals.
Limitations of BMI
BMI is a useful screening tool but has important limitations you should be aware of:
- Muscle mass: Athletes and very muscular individuals often have a high BMI despite low body fat. A bodybuilder could be classified as "obese" by BMI alone.
- Age: Older adults tend to have more body fat at the same BMI compared to younger people. BMI may underestimate obesity risk in the elderly.
- Sex differences: Women typically have more body fat than men at the same BMI. BMI does not adjust for this.
- Ethnicity: Health risks associated with a given BMI vary across ethnic groups.
- Distribution: Where you carry fat matters. Abdominal fat (waist circumference) is a stronger predictor of cardiovascular risk than BMI alone.
⚠️ BMI is a screening tool, not a diagnosis. Always consult a qualified healthcare professional for a comprehensive health assessment.
How to Improve Your BMI
If your BMI falls outside the healthy range, gradual, sustainable lifestyle changes are more effective than rapid interventions:
- Balanced nutrition: Focus on whole foods, vegetables, lean proteins and healthy fats. Reduce processed foods and added sugars.
- Regular physical activity: Aim for at least 150 minutes of moderate aerobic activity per week (WHO recommendation), plus strength training twice per week.
- Adequate sleep: Poor sleep is linked to weight gain. Adults need 7–9 hours per night.
- Stress management: Chronic stress elevates cortisol, which promotes fat storage especially around the abdomen.
- Professional guidance: A registered dietitian or certified fitness trainer can provide a personalised plan far more effective than generic advice.
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Open BMI Calculator →Frequently Asked Questions
Q: What is a healthy BMI for adults in India?
A: Using WHO Asian criteria, 18.5–22.9 is normal, 23–24.9 is overweight (at risk), and 25+ is obese for Asian adults.
Q: Can BMI be accurate for children?
A: Yes, but child BMI uses age and sex-specific percentile charts (BMI-for-age), not the adult ranges shown above.
Q: Is BMI the same as body fat percentage?
A: No. BMI is calculated from height and weight only. Body fat percentage requires additional measurements like skinfold thickness, DEXA scan, or bioelectrical impedance.
Q: How often should I check my BMI?
A: For most healthy adults, once every 6–12 months is sufficient unless you are actively managing your weight, in which case monthly tracking is useful.