BMI Calculator

About BMI Calculator

BMI is weight in kilograms divided by height in metres squared. At 172 cm and 70 kg that is 70 ÷ 1.72² = 70 ÷ 2.9584 ≈ 23.7. Under the WHO table that is a normal weight. A health check in Seoul would label the same person “pre-obese”. Both are correct; they are using different cut-offs.

This calculator lets you switch between the standard WHO classification, the Korean Society for the Study of Obesity (KSSO) 2022 table, which follows the lower Asian cut-offs, and the Japan Society for the Study of Obesity (JASSO) table. Besides the BMI and its category it shows a reference weight at BMI 22, the healthy weight range for the chosen table, and how many kilograms separate you from that range. If you use pounds and feet, it converts them first: 5 ft 10 in and 180 lb become 177.8 cm and 81.6 kg, a BMI of 25.8. It is meant for adults (20 and over), and a line under the result says so; it does not compute percentiles for children or teens.

An index built by a statistician

The idea of dividing weight by height squared goes back to the Belgian mathematician Adolphe Quetelet in the 1830s. He noticed that adult weight tends to scale with the square of height, and he used it to describe the “average man”, not to diagnose anybody. For more than a century it was known simply as the Quetelet index.

The name “body mass index” came from a 1972 paper by the physiologist Ancel Keys and colleagues. They compared several weight-for-height formulas across samples of men from different countries and found that weight divided by height squared tracked measured body fat best while staying largely independent of height. Keys recommended it for population studies and was openly skeptical of using it to judge individuals. That is still the fairest description of what BMI does well: sorting large groups cheaply, not describing one body precisely.

Why Asian cut-offs are lower

The WHO table puts overweight at 25 and obesity at 30. In 2004 a WHO expert consultation reviewed evidence that many Asian populations carry a higher percentage of body fat at the same BMI than Europeans, and that a substantial share of Asian adults already face high risk of type 2 diabetes and cardiovascular disease below BMI 25. Rather than impose one new set of Asian cut-offs, the consultation kept the international categories and added public health action points at 23, 27.5, 32.5 and 37.5, leaving countries to decide how to use them.

Korea's obesity society chose 23 and 25: normal is 18.5–22.9, pre-obese 23–24.9, class 1 obesity 25–29.9, class 2 30–34.9 and class 3 from 35. That is the table behind this calculator's Korean option. Japan (JASSO) uses 25 as the start of obesity, with class 1 at 25–30, class 2 at 30–35, class 3 at 35–40 and class 4 from 40, and no overweight band; that is the calculator's Japan option. Singapore, India and others have their own versions. For readers elsewhere the practical point is that obesity rates quoted from Asian countries are not directly comparable with US or European figures, and that if you are of Asian ancestry your clinician may reasonably use a lower threshold than the WHO default. The CDC and NIH in the US still use 25 and 30 for adults in general.

People BMI misjudges

BMI knows your weight, not what the weight is made of. The misreadings are predictable:

• Muscular people. A 180 cm, 90 kg rugby player or lifter has a BMI of 27.8, which is overweight by WHO and class 1 obese by the Korean table, even with low body fat.
• Older adults. Muscle declines and fat rises with age, so an unchanged BMI can hide more body fat. Loss of height from spinal compression nudges BMI up as well.
• “Normal-weight” central obesity. Thin limbs and a large belly can produce a normal BMI with plenty of visceral fat. Waist circumference is there to catch this.
• Children and teens. BMI changes naturally during growth, so fixed adult cut-offs do not apply. The CDC compares a child with others of the same age and sex: below the 5th percentile is underweight, the 85th to below the 95th is overweight, and the 95th percentile or above is obesity.
• Pregnancy, where weight gain is judged against separate guidance based on pre-pregnancy BMI.

Measure your waist too

Fat stored inside the abdomen is more closely tied to metabolic disease than total weight. A WHO expert consultation on waist circumference describes increased risk above 94 cm for European men and 80 cm for European women, and substantially increased risk above 102 cm and 88 cm. Korea defines abdominal obesity as 90 cm or more for men and 85 cm or more for women. Japan reverses the sexes, 85 cm for men and 90 cm for women, because its thresholds were set to match a visceral fat area of 100 cm² on CT scans.

Measure midway between the lowest rib and the top of the hip bone, after a normal breath out. A man with BMI 23.5 and a 92 cm waist may well have a worse metabolic picture than one with BMI 26 and an 82 cm waist.

Where the BMI 22 reference weight comes from

The reference weight shown here is height (m)² × 22. The figure comes from Japanese epidemiology, where BMI 22 was associated with the lowest rates of hypertension, diabetes and dyslipidaemia, and it is still how Japan's health ministry defines standard weight. It is a single point, so treat the healthy range as the real target. The percentage figure is simply your weight divided by that reference weight.

Worked examples

How to use it

1. Pick units (cm/kg or ft-in/lb) and enter height and weight.
2. Choose the WHO table, the Korean (Asian) table or the Japanese (JASSO) table.
3. Read the BMI, the category, the healthy range and the gap to it.

Frequently asked questions

Notes

BMI is a screening measure, not a diagnosis. Decisions about weight treatment belong with a clinician.

Related guides

• BMR and TDEE Formulas: A Century of Equations, How Accurate They Are, and Why the 3,500-Calorie Rule Misleads