What BMI actually measures
BMI is your weight in kilograms divided by height in meters squared (kg/m²). It was designed as a population screening tool, not a diagnosis. The WHO categories — underweight <18.5, normal 18.5–24.9, overweight 25–29.9, obese 30+ — correlate with health risk at the population level, but they say nothing about your body composition.
Know its limits
- Muscle mass: athletes with high muscle can show "overweight" while being extremely lean. BMI can't tell muscle from fat.
- Age: the same BMI carries different risk at 25 vs 75. Slightly higher BMI (25–27) is associated with better outcomes in older adults.
- Ethnicity: WHO categories are based mostly on European data. For East and South Asian populations, health risks start at lower BMIs (~23), and many guidelines use lower cutoffs.
- Children: BMI categories differ by age and sex — kids should be plotted on percentile growth charts, not adult cutoffs.
- Distribution: where fat sits matters. Waist-to-height ratio (keep it under 0.5) and body-fat percentage often add more signal for individuals.
Better quick checks
If you're worried about your numbers, waist-to-height ratio is a free, more informative screen: measure your waist, divide by your height (same units), and aim below 0.5. For body composition, a DEXA scan or even a consistent tape-measure trend beats any calculator.