What is BMI?
Body Mass Index (BMI) is a simple calculation that uses your height and weight to estimate whether you're at a healthy weight. It's one of the most widely used screening tools by doctors and health organizations worldwide, including the World Health Organization (WHO).
BMI is not perfect — it doesn't measure body fat directly or distinguish between muscle and fat — but it's a quick, free way to get a baseline understanding of your weight status.
BMI Categories
| BMI Range | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 – 24.9 | Normal weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obese |
How BMI is calculated
The BMI formula is straightforward:
Example: If you weigh 70 kg and are 1.75 m tall, your BMI is 70 ÷ (1.75 × 1.75) = 22.9, which falls in the "Normal weight" range.
For imperial units, the formula is BMI = (weight in lb × 703) ÷ height² (in inches).
Limitations of BMI
BMI is a screening tool, not a diagnostic one. It has several known limitations:
- Doesn't distinguish muscle from fat. Athletes and bodybuilders often have a "high" BMI despite being very healthy.
- Doesn't account for body composition. Two people with the same BMI can have very different amounts of body fat.
- May not apply equally to all ethnicities. Some studies suggest South Asian populations face health risks at lower BMI thresholds than the standard WHO categories.
- Less reliable for older adults. Muscle mass naturally decreases with age, which can skew results.
- Not appropriate for children. Pediatric BMI uses age- and sex-specific percentiles instead.
When to consult a doctor
If your BMI falls outside the normal range, consider speaking with a healthcare professional. They can assess your overall health using additional measures like waist circumference, body fat percentage, blood pressure, blood sugar, and cholesterol — giving a much more complete picture than BMI alone.