Body Mass Index (BMI) is a simple calculation used to estimate whether a person's weight falls within a healthy range based on their height. It's widely used in clinical and public health settings as a quick screening tool.
BMI doesn't measure body fat directly, but it provides a general indication of weight status.
The BMI Formula
Step-by-Step Example
Weight: 70 kg · Height: 1.70 m (170 cm)
Step 2 — Divide weight by height²: 70 ÷ 2.89
Step 3 — Result: BMI = 24.22
BMI Categories (General Adult Guidelines)
| Category | BMI Range |
|---|---|
| ● Underweight | < 18.5 |
| ● Normal weight | 18.5 – 24.9 |
| ● Overweight | 25 – 29.9 |
| ● Obesity | ≥ 30 |
BMI Considerations by Age and Sex
While the standard BMI categories apply broadly to adults, interpretation can vary based on age and biological sex.
Adults (18–64 years)
- Same BMI categories for men and women
- Men tend to have more lean mass; women tend to have higher body fat at the same BMI
- A BMI of 25 in a man may reflect more muscle; in a woman, it may reflect slightly higher fat mass
Older Adults (65+ years)
- Slightly higher BMI ranges may be acceptable: Normal ~23–29 (context-dependent)
- Lower BMI (<22) may be associated with undernutrition or frailty
- Being slightly above "normal" BMI can be protective in this group
Children & Adolescents (2–17 years)
BMI is interpreted differently using percentiles based on age and sex:
| Category | Percentile |
|---|---|
| Underweight | < 5th percentile |
| Healthy weight | 5th – 85th percentile |
| Overweight | 85th – 95th percentile |
| Obesity | > 95th percentile |
Final Note
BMI is a useful starting point, but it has real limitations. It does not distinguish between fat mass and lean muscle mass. A bodybuilder may show an "obese" BMI despite having very low body fat.
For a more accurate assessment, BMI should be used alongside other measurements such as body fat percentage, waist circumference, and overall clinical evaluation. Pair it with the BMR Calculator and TDEE Calculator for a more complete picture.