Calculate BMI

What is BMI

BMI, also known as Body Mass Index, is a body mass index that indicates the degree of obesity of a person calculated from height and weight.

Obesity increases the risk of diabetes and high blood pressure.Underweight can also be caused by malnutrition or chronic progressive disease.

Knowing the degree of obesity is useful for disease prevention and treatment.

BMI formula

BMI is calculated using the formula below.

BMI = Body weight(kg) ÷ (Height(m))2

Standard weight = (Height(m))2 x 22

BMI criteria


The formula for calculating BMI is universal, but the criteria for obesity differ from country to country.

The Japan Obesity Society considers a BMI of 22 to be the normal weight, which is the ideal weight to prevent illness.

Obesity is defined as 25 or more, severe obesity is defined as 35 or more, and underweight is defined as less than 18.5.

Standards of the Japan Obesity Society

BMI index Condition
< 18.5 Underweight (thinness)
18.5 – 24.9 Normal range
25.0 – 29.9 Obese (Class I)
30.0 – 34.9 Obese (Class II)
35.0 – 39.9 Obese (Class III)
≥ 40.0 Obese (Class IV)

World Health Organization (WHO) standards

BMI index Condition
< 16.0 Severe thinness
16.0 – 16.9 Moderate thinness
17.0 – 18.4 Mild thinness
18.5 – 24.9 Normal range
25.0 – 29.9 Obesity
30.0 – 34.9 Obese (Class I)
35.0 – 39.9 Obese (Class II)
≥ 40.0 Obese (Class III)


For infants from 3 months old to 5 years old, the degree of obesity is expressed by the Kaup index.

This is the same as an adult's BMI, and is calculated by the formula "weight (kg) ÷ (height (m))2".

Calculation of infant obesity (Kaup index)

Children/students (elementary school students/junior high school students)

In the case of children and students, the Laurel index expresses the degree of obesity.

The Laurel index is calculated using the formula "weight (kg) ÷ (height (m))3 x 3".

Calculation of the degree of obesity (Laurell index) of children and students