What is BMI (Body Mass Index)
BMI is a simple ratio of weight to height squared, used for a quick preliminary assessment of whether a person's weight falls within a healthy range.
Calculation formula
BMI = weight(kg) / height(m)²
For example, at 75 kg and 1.78 m: BMI = 75 / (1.78 × 1.78) ≈ 23.7
BMI categories on the WHO scale
- Below 18.5: underweight — may indicate nutritional deficiency; consult a doctor.
- 18.5–24.9: normal weight — associated with the lowest health risk in population studies.
- 25–29.9: overweight — increased risk of cardiovascular disease and type 2 diabetes.
- 30 and above: obese — significantly increased risk of chronic disease; medical consultation recommended.
Why BMI is a guide, not a diagnosis
BMI is a simple screening measure that does not account for body composition. Athletes and people with high muscle mass may show an elevated BMI despite having low body fat. For example, a professional weightlifter with a BMI of 28 may have less body fat than a sedentary person with a BMI of 23.
BMI also doesn't account for fat distribution — belly fat (visceral fat) poses a greater health risk than fat on the hips or thighs, even at the same BMI. For a fuller picture, consider waist circumference, waist-to-hip ratio, or body fat percentage (measured via bioimpedance or calipers).
Special cases
- Children and teens: age- and sex-specific BMI percentile charts are used instead of the adult scale.
- Pregnancy: BMI is not used to assess weight during pregnancy — doctors use different weight-gain guidelines.
- Older adults: a slightly higher BMI (25-27) in seniors is sometimes associated with better health outcomes than the "ideal" range.
- Different ethnic groups: some studies show health risks may begin at different BMI thresholds depending on ethnic background.
The result is an approximate indicator, not a medical diagnosis. For a full health assessment, consult a doctor who can factor in body composition, waist circumference, lab tests, and family history.
FAQ
What is BMI?
BMI (Body Mass Index) is a measure of the ratio between weight and height, used as a rough screening tool to estimate whether a person's weight is within a healthy range.
What are the normal BMI ranges?
According to the WHO classification: below 18.5 is underweight, 18.5-24.9 is normal, 25-29.9 is overweight, and 30 or above is obese.
Does BMI account for muscle mass?
No, BMI doesn't distinguish between muscle and fat mass. Athletes with high muscle mass may show an elevated BMI despite having low body fat.
Can I fully trust BMI?
BMI is a useful screening tool, but not a diagnosis. For a complete health picture, consider body composition, waist circumference, and a doctor's consultation.