The formula of body mass index published by the World Health Organization (WHO) is: body mass index (BMI) = weight (kg)/ height (m) 2.
For example, a person's height is 1.75m and his weight is 68kg, and his body mass index =68/( 1.75)2=22.2 (kg/m2).
When the body mass index is 18.5 ~ 24.9, it is normal.
Body mass index < 18.5 is emaciation;
Body mass index ≥25.0 is overweight; Obesity with body mass index between 25.0 and 29.9 is1; Body mass index > 30.0 ~ 34.9 is grade 2 obesity; Body mass index > 35.0 ~ 39.9 is grade 3 obesity; Body mass index > 40.0 is grade 4 obesity.
For example, your height is 1.65m, your weight is 50kg and your body mass index is 50/(1.65) 2 =18.36 (kg/m2).
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