BMI: what the number means, where the categories come from, and what it misses
2026-09-22
BMI is weight in kilograms divided by height in metres squared. It was designed in the 1830s to describe populations, not individuals, which is why it is useful for screening and useless for telling a rugby player from a couch potato of the same size.
Open the calculator
The WHO categories
- Under 18.5 β underweight
- 18.5β24.9 β normal
- 25β29.9 β overweight
- 30β34.9 β obesity class I
- 35β39.9 β class II
- 40+ β class III
Why Korea and Japan use lower cut-offs
East Asian populations show higher rates of diabetes and heart disease at the same BMI, so Korean and Japanese guidelines set overweight at 23 and obesity at 25. The calculator applies the Korean standard on the Korean site and WHO elsewhere.
What to check alongside BMI
- Waist circumference: over 90 cm (men) or 85 cm (women) in Korean guidelines, 102/88 cm in WHO, signals visceral fat regardless of BMI
- Body-fat percentage from a scale or DEXA scan for athletes and older adults
- Trend over time matters more than a single reading