BMI Formula and Obesity Classification: What BMI Counts as Obese? Plus Target BMI by Age

The BMI formula and how to calculate it

BMI (Body Mass Index) is calculated as “weight (kg) ÷ height (m) ÷ height (m).” The key is to convert height to meters, not centimeters: for 170 cm, divide by 1.7 twice. Dividing by the square of the height gives the same result.

1. 1

   Convert height to meters

   Divide by 100, as in 170 cm → 1.7 m or 158 cm → 1.58 m.

2. 2

   Divide weight by height twice

   For 170 cm and 65 kg, 65 ÷ 1.7 ÷ 1.7 = 22.49… On a phone calculator, just press “65 ÷ 1.7 ÷ 1.7 =” in a row.

3. 3

   Read it to one decimal place

   BMI is usually shown to one decimal place. 22.49… becomes 22.5, which is classified as “normal weight.”

Dividing by the square of the height evens out the effect that taller people naturally weigh more. Dividing by height only once makes the value larger for taller people, while dividing three times makes it larger for shorter people. For adult body sizes, dividing by the square is considered least affected by height, which is why it is used as an international indicator.

Japan's obesity classification (Japan Society for the Study of Obesity)

In Japan, BMI is divided into six categories based on the criteria of the Japan Society for the Study of Obesity. Health checkups and specific health checkups in Japan also use this standard.

A BMI of 35 or more is also called “severe obesity.” Note that “obesity” and “obesity disease” are different in Japan's guidelines. Obesity refers to the body-size state of a BMI of 25 or more, while obesity disease means obesity plus health problems caused by it, such as diabetes or high blood pressure (or a large amount of visceral fat), so that weight loss is medically necessary.

How it differs from the WHO standard: why Japan starts “obese” at 25

Under the international standard of the World Health Organization (WHO), a BMI of 25 to under 30 is “pre-obese” (overweight), and “obese” starts at 30. Japan's standard is stricter, classifying people as obese from a value 5 points lower than the WHO.

Asian populations, including Japanese people, tend to accumulate visceral fat more easily than people of European descent at the same BMI, and conditions such as diabetes and high blood pressure are known to increase from around a BMI of 25. That is why Japan uses 25 as the cutoff for obesity. So even if an overseas study or app says “a BMI of 27 is overweight,” the same value counts as obesity (class 1) under Japan's standard. Several other Asian countries also use lower cutoffs than the WHO, so check the standard used where you live.

Standard weight (BMI 22) and degree of obesity (%)

Working the formula backward gives you the weight that matches a target BMI. A common one is the “standard weight” at BMI 22, calculated as height (m) × height (m) × 22. It is based on a study of Japanese people in which the BMI with the fewest illnesses was 22.2 for men and 21.9 for women.

The difference between your current weight and the standard weight, expressed as a percentage, is the “degree of obesity”: (weight − standard weight) ÷ standard weight × 100. At 175 cm and 80 kg, the difference from the standard weight of 67.4 kg is 12.6 kg, so the degree of obesity is +18.7%. At 155 cm and 45 kg, you are 7.9 kg lighter than the standard weight of 52.9 kg, a degree of obesity of −14.9%. A list of standard and “beauty” weights by height is in the standard weight chart by height.

Target BMI by age

Japan's Ministry of Health, Labour and Welfare, in its “Dietary Reference Intakes for Japanese (2020),” changes the target BMI range by age. The upper limit is 24.9 for every age group, but the lower limit rises with age.

The lower limit rises with age to prevent the loss of muscle strength from being too thin, and frailty, the weakened state of body and mind just before someone needs care. For people 65 and over, even a BMI of 20, which would be normal for younger people, is treated as “below target.” In middle-aged and older dieting, not losing too much is just as important.

What BMI alone can't tell you

• It can't distinguish muscle from fat: muscular athletes can have a BMI over 25 and show up as “obese” even with little body fat.
• It can't show where fat is stored: some people have a normal BMI but carry visceral fat around the abdomen, sometimes called “hidden obesity.” This is why Japan's specific health checkups measure waist size (85 cm or more for men, 90 cm or more for women).
• It doesn't apply to children: during growth the balance of height and weight changes with age, so the Kaup index is used for infants and the Rohrer index or degree of obesity for schoolchildren. See how to calculate the Kaup and Rohrer indexes for details.
• Pregnancy has separate guidelines: weight management during pregnancy is based on recommended weight gain derived from your pre-pregnancy BMI.

BMI is quick and handy as a rough indicator of body size, but it does not represent your health itself. The basic approach is to look at it together with blood pressure, blood sugar, waist size, and other results.

Calculate BMI and healthy weight together

Enter your height and weight in the “BMI & healthy weight” tab of the BMI & Healthy Weight Calculator to see your BMI, obesity classification, difference from standard weight, and normal-weight range right away. You can enter height as “160,” “160cm,” or “1.6m.” If you enter your age, it also shows whether you are within the target BMI for your age group.

If you want to lose weight, check your daily calorie expenditure in the “BMR & calories” tab, then plan toward your goal in the “Target weight & diet” tab to see how many kcal to cut per day. The thinking behind it is explained in the basal metabolic rate formula and daily calorie expenditure and how many kilograms you can lose in a month.

FAQ

What is the BMI formula?

BMI = weight (kg) ÷ height (m) ÷ height (m). Convert height from centimeters to meters and divide twice. At 170 cm and 65 kg, 65 ÷ 1.7 ÷ 1.7 = 22.5; at 160 cm and 55 kg, it is 21.5. Both are normal weight.

What BMI counts as obese?

Under the WHO international standard, 25 to under 30 is “pre-obese” (overweight) and obesity starts at 30. Japan's standard is stricter: under 18.5 is underweight, 18.5 to under 25 is normal, 25 to under 30 is obesity class 1, 30 to under 35 is class 2, 35 to under 40 is class 3, and 40 or more is class 4.

At 160 cm, what weight puts BMI over 25?

At 160 cm, BMI is exactly 25 at 64.0 kg. Because BMI is rounded to one decimal place, even 63.9 kg (BMI 24.96) is shown as 25.0 and classified as obesity class 1 under Japan's standard. At 170 cm, BMI 25 is 72.3 kg; at 155 cm, it is 60.1 kg.

If my BMI is normal, can I assume I'm healthy?

BMI alone can't tell you that. Some people with a normal BMI carry a lot of visceral fat (“hidden obesity”). Conversely, muscular people can have a BMI over 25 with little body fat. Look at it together with your waist size, blood pressure, blood sugar, and other checkup results.