How BMI is calculated, and why WHO and Japan use different thresholds
BMI (Body Mass Index) is the standard rough indicator of body composition. The “BMI 25 = obese” line you may have seen is actually different between international (WHO) and Japanese standards. This article unpacks the formula and the cross-cultural cutoff difference.
The formula: weight ÷ height²
BMI is straightforward:
BMI = weight (kg) ÷ height (m)²For someone 170 cm tall weighing 65 kg:
BMI = 65 ÷ (1.70)² = 65 ÷ 2.89 ≈ 22.49Use meters for height. Plugging cm in as-is squares to a number 10,000 times too large and gives an answer near zero.
Why “height squared” rather than cubed
BMI traces back to 1832 and the Belgian statistician Adolphe Quetelet (“Quetelet Index”), originally designed as a statistical indicator of obesity in populations, not as a clinical metric.
Weight grows with height, but the population data fits a square law better than a cube law, even though body volume scales with cube. The reason: humans are not perfect scale models — taller bodies tend to be relatively narrower. Cubing penalizes tall people unfairly; not scaling at all favors them. Squaring is the empirical sweet spot.
In other words, the ² in BMI is a statistical fit, not a physical necessity. This is also why BMI norms vary across populations.
WHO international thresholds
WHO classifies adult BMI as:
| BMI range | Classification |
|---|---|
| Below 16.0 | Severe thinness |
| 16.0 to below 17.0 | Moderate thinness |
| 17.0 to below 18.5 | Mild thinness |
| 18.5 to below 25.0 | Normal weight |
| 25.0 to below 30.0 | Pre-obese (overweight) |
| 30.0 to below 35.0 | Obese class I |
| 35.0 to below 40.0 | Obese class II |
| 40.0 and above | Obese class III |
Under WHO, obesity starts at 30.0. The 25.0–30.0 band is “overweight / pre-obese”.
The Japanese standard
The Japan Society for the Study of Obesity uses a different threshold:
| BMI range | Classification |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to below 25.0 | Normal weight |
| 25.0 to below 30.0 | Obesity (level 1) |
| 30.0 to below 35.0 | Obesity (level 2) |
| 35.0 to below 40.0 | Obesity (level 3) |
| 40.0 and above | Obesity (level 4) |
Obesity starts at 25.0, five points lower than WHO.
Why Japan uses the stricter cutoff
It’s not “Japanese people are thinner, so the bar is set lower”. The threshold comes from disease-risk epidemiology, not aesthetics.
Japanese cohort studies showed that:
- Risk of type 2 diabetes, hypertension, and dyslipidemia rises sharply once BMI passes 25.
- At the same BMI, Japanese populations tend to accumulate more visceral fat than European/American populations, raising metabolic syndrome risk earlier.
- East Asian populations generally have higher body fat percentage at a given BMI.
Together, these data put the “risk inflection point” around BMI 25 for Japanese adults. WHO has separately recommended 23 / 27.5 as additional action points for Asian populations (WHO Expert Consultation, 2004).
Why Japan considers 22 “ideal”
Within Japan, “BMI 22 is ideal” has become folk wisdom. The basis:
- Disease-risk epidemiology shows the lowest morbidity around BMI 22.
- “Lowest” within the 18.5–25 band; the differences inside the normal range are small.
So “must hit BMI 22 or you’re unhealthy” is overstated. As long as you’re inside 18.5–25, the differences within the band are limited.
Limits of BMI
BMI uses only weight and height. It cannot distinguish:
- Muscle vs fat — high-muscle athletes can read as obese without being unhealthy.
- Fat distribution — visceral and subcutaneous obesity carry very different risks.
- Age and sex — body composition at the same BMI varies between elderly and younger adults, between men and women.
Clinical practice combines BMI with waist circumference, body fat percentage, and blood markers. BMI is a screening indicator, not a diagnostic tool on its own.
Choosing a standard
| Context | Recommended threshold |
|---|---|
| Within Japan / Japanese population | Japan standard (obesity at 25) |
| International comparison | WHO standard (obesity at 30) |
| Personal sanity check | Are you inside 18.5–25? |
The BMI calculator on this site shows results against both standards at once. For someone in the 25–30 band, where the two scales disagree, that comparison is informative.