BMI Calculator (Japanese / Asian Standard) — Your BMI, the Japanese weight category and your standard weight

How BMI is calculated and classified in Japan

BMI is your weight in kilograms divided by your height in meters squared. For 170 cm and 65 kg that is 65 ÷ (1.7 × 1.7), about 22.5. Remember to convert height to meters first.

The Japan Society for the Study of Obesity uses these categories, and they are the ones printed on the annual health check (kenko shindan) results you receive from an employer or city:

• Under 18.5: underweight
• 18.5 to under 25: normal weight
• 25 to under 30: obese, class 1
• 30 to under 35: obese, class 2
• 35 to under 40: obese, class 3
• 40 and over: obese, class 4

Why Japan draws the line at 25

The WHO defines obesity as a BMI of 30 or more, and many Western countries call 25 to 30 "overweight". Japan, like several other East Asian countries, uses 25 because East Asian populations tend to develop diabetes, high blood pressure and other lifestyle diseases at lower BMIs than Western populations.

This is why a result that reads "normal" on a site from your home country can read "obese (class 1)" on a Japanese health check. Neither is wrong; they are different scales for different populations. Japanese employers and clinics use the Japanese one.

Why "standard weight" means BMI 22

Standard weight (hyojun taiju) is height in meters squared times 22. At 170 cm that is about 63.6 kg.

The 22 is not an aesthetic target. It comes from studies that found the lowest rates of diabetes, hypertension and lipid disorders around that value. Treat it as the point of lowest statistical risk, not a number you must reach.

What BMI cannot tell you

BMI is easy to calculate but says nothing about what your weight is made of. It can be misleading for:

• Muscular people: muscle is denser than fat, so BMI can read high with little body fat.
• Fat distribution: two people with the same BMI can carry fat very differently, and BMI cannot see visceral fat. Japanese health checks add a waist measurement for this reason.
• Older adults: muscle loss with age changes body composition even when weight stays the same.
• Children and teenagers: the categories are for adults aged 18 and over.
• Pregnancy: separate weight-gain guidelines apply.

Reading the number sensibly

BMI does not move much day to day. Weight varies by around a kilogram with meals, fluids and the time of day, so a trend measured under the same conditions is more useful than any single reading.

Rapid weight loss is hard on the body. If your category concerns you, avoid extreme dieting and discuss the result together with your health-check report with a doctor or dietitian.