Body Mass Index (BMI) is a simple calculation using height and weight: BMI = weight (kg) ÷ height (m)². It provides a quick screening metric for weight categories.
BMI Categories
- Below 18.5 — Underweight
- 18.5–24.9 — Normal weight
- 25.0–29.9 — Overweight
- 30.0+ — Obese
Limitations
BMI does not distinguish between muscle and fat. A muscular athlete may have a "overweight" BMI despite low body fat. It also does not account for age, sex, ethnicity, or body composition. BMI is best used as a population-level screening tool, not a definitive health diagnosis.
Why BMI Is Still Widely Used
Despite its limitations, BMI remains the standard population-level screening tool because it's free, requires no equipment beyond a scale and tape measure, and correlates reasonably well with health risk across large groups — even though it can mislabel individuals. The WHO and CDC both use BMI as a first-pass screen, then recommend follow-up assessment (body fat %, waist circumference) for borderline or high-risk cases.
BMI for Children and Teens
Adult BMI categories don't apply to children — pediatric BMI is plotted on age- and sex-specific growth charts as a percentile, since body composition changes dramatically during growth. A BMI that's "overweight" for an adult could be perfectly normal for a growing teenager.