Understanding your Body Mass Index
Body Mass Index (BMI) is a simple ratio that estimates whether your weight is in a healthy range for your height. Clinicians use it worldwide as a fast first-pass screen because it needs only two numbers you already know. It doesn’t measure body fat directly — but across large populations it correlates well enough to be a useful starting point.
The formula
The calculation is the same everywhere; only the units differ:
- Metric:
BMI = weight(kg) / height(m)² - Imperial:
BMI = weight(lb) / height(in)² × 703
For example, someone 1.75 m tall weighing 70 kg has a BMI of
70 / (1.75 × 1.75) ≈ 22.9 — comfortably within the normal range.
BMI categories (adults 20+)
| BMI | Category | General note |
|---|---|---|
| Below 18.5 | Underweight | May indicate undernutrition; worth a check-up. |
| 18.5 – 24.9 | Normal | Lowest associated risk for most adults. |
| 25.0 – 29.9 | Overweight | Raised risk; lifestyle factors matter. |
| 30.0 and above | Obese | Higher risk of related conditions. |
Ranges follow the World Health Organization classification for adults. Different cut-offs apply to children, and some health bodies use lower thresholds for certain populations.
What BMI can’t tell you
Because it only uses height and weight, BMI can’t distinguish muscle from fat or tell you where fat is stored. A strength athlete may register as “overweight” with very low body fat, while someone with a normal BMI can still carry unhealthy abdominal fat. Measures like waist circumference, the waist-to-height ratio and body-fat percentage add important context.