BMI and Ideal Weight

What is BMI?

BMI (Body Mass Index) is a body size index that indicates obesity levels, calculated based on weight and height. While a different index exists for children, BMI is used as an international standard for adults. It is important to monitor your BMI regularly to maintain good health.

Calculation Formula

• BMI = Weight (kg) ÷ (Height m)²


• Ideal Weight = (Height m)² × 22

Classification Criteria

Although the BMI formula is universal, the criteria for determining obesity vary by country.

• Adults

Classification Criteria of the Japan Society for the Study of Obesity BMI Value Classification Less than 18.5 Underweight (Thin) 18.5 to less than 25 Normal Weight 25 to less than 30 Obese (1 level) 30 to less than 35 Obesity (2 level) 35 to less than 40 Obesity (3 degree) 40 or higher Obesity (4 degree)

World Health Organization (WHO) Classification Criteria BMI Value Classification Below 16 Severely underweight 16.00–16.99 (inclusive) Underweight 17.00–18.49 (inclusive) Slightly underweight 18.50–24.99 (inclusive) Normal weight 25.00–29.99 (inclusive) Pre-obesity 30.00–34.99 (inclusive) Obesity (1 degree) 35.00–39.99 (inclusive) Obesity (2 degree) 40.00 and above Obesity (3 degree)

• Children

The Kaup index is used for infants, and the Rohrer index is used for school-age children. Please refer to “Childhood Obesity”.

• Pregnant Women

Obesity or being underweight during pregnancy increases the risk of illness and other complications. Additionally, care must be taken to avoid both excessive and insufficient weight gain during pregnancy.

For information on the recommended amount of weight gain based on your pre-pregnancy BMI, please refer to “Guidelines for Weight Gain During Pregnancy” (Japan Society of Obstetrics and Gynecology) .

Healthy Weight

The Japan Society for the Study of Obesity defines a BMI of 22 as a healthy weight (standard weight), which is statistically associated with the lowest risk of disease. 25 or higher is classified as obesity, and below 18.5 as underweight.

Risks of Obesity and Underweight Conditions

Obesity increases the risk of lifestyle-related diseases such as diabetes, high blood pressure, and dyslipidemia. Underweight conditions, on the other hand, can lead to malnutrition and chronic progressive diseases. (Japanese Society for the Study of Obesity)

In pregnant women, obesity (BMI of 25 or higher) increases the risk of gestational hypertension, gestational diabetes, and the incidence of macrosomia and cesarean sections; conversely, being underweight (BMI below 18.5) increases the risk of threatened preterm labor, preterm birth, and giving birth to a low birth weight infant. (Ministry of Health, Labor and Welfare)

Relationship with Metabolic Syndrome

Since BMI and visceral fat do not necessarily correlate, BMI is not included in the diagnostic criteria for metabolic syndrome; however, BMI is used in the criteria for specific health checkups and specific health guidance to identify individuals at risk of developing metabolic syndrome.

History

Adolphe Quetelet of Belgium proposed the BMI in 1835 by applying statistical methods. Although BMI has its limitations—since fat and muscle mass vary even among people of the same weight—it is now used universally around the world due to its simple calculation formula.

*This site assumes no responsibility whatsoever for calculation results or other information. Additionally, we do not provide individual consultations.