Definition

BMI (Body Mass Index) is a measure of body fatness calculated from weight and height. The formula is weight (kg) divided by height (m) squared. It was devised in 1832 by Belgian statistician Adolphe Quetelet as a simple screening tool for population-level obesity assessment.

Classification Criteria

Under WHO standards, a BMI below 18.5 is underweight, 18.5-24.9 is normal, 25.0-29.9 is overweight, and 30.0 or above is obese. The Japan Society for the Study of Obesity classifies 25.0 and above as obese, applying a stricter threshold than the WHO.

WHO classification compared with the Japan Society for the Study of Obesity classification at each BMI level
BMIWHO classificationJapanese classification
Below 18.5UnderweightUnderweight
18.5 - 24.9NormalNormal weight
25.0 - 29.9OverweightObesity class 1
30.0 - 34.9ObeseObesity class 2
35.0 - 39.9ObeseObesity class 3
40.0 and aboveObeseObesity class 4
The two systems disagree over one band: 25.0 to 29.9. A BMI of 26 is merely overweight by WHO standards, yet already counts as obesity in Japan. That single shift explains why an international health article and a Japanese medical check-up can describe the identical number in very different words. What is distinctive about the Japanese system is where obesity starts - at 25.0 - and it treats 35.0 and above as severe obesity.
Body weight at the BMI 18.5, 25.0 and 30.0 boundaries for each height
HeightBMI 18.5BMI 25.0BMI 30.0
150 cm41.6 kg56.3 kg67.5 kg
155 cm44.4 kg60.1 kg72.1 kg
160 cm47.4 kg64.0 kg76.8 kg
165 cm50.4 kg68.1 kg81.7 kg
170 cm53.5 kg72.3 kg86.7 kg
175 cm56.7 kg76.6 kg91.9 kg
180 cm59.9 kg81.0 kg97.2 kg
These are the same boundaries read backwards, using weight = BMI multiplied by height in metres squared. At 170 cm, dropping below 53.5 kg means underweight, passing 72.3 kg means obese under the Japanese threshold, and passing 86.7 kg means obese under the WHO threshold as well. Because 5 cm of height moves each boundary by roughly 3 kg, comparing your weight in kilograms against someone else's tells you almost nothing.

Limitations and Caveats

BMI cannot distinguish between muscle mass and fat mass. A muscular person may have a high BMI with low health risk. Additionally, the same BMI can carry different risks depending on whether fat is visceral or subcutaneous. It remains a screening tool and should not be used as a definitive health assessment.

Global Ranking Context

MyRank uses BMI to show where you stand relative to the global population. Understanding that BMI thresholds vary by region helps you interpret your ranking more accurately and motivates informed health decisions.