Why BMI 23.5 Is Normal in Geneva and Overweight in Seoul

One number, five verdicts

Take someone 170 cm tall who weighs 68 kg. Their BMI is 68 ÷ 1.70² = 23.5. Type that into a calculator built on the World Health Organization's international table and the verdict is «normal weight». Type it into a Korean hospital's check-up form and it reads «pre-obese». An Indian clinic would call it overweight. A Japanese one, normal. The U.S. Centers for Disease Control would say normal — while the American Diabetes Association would say this person should already be screened for diabetes.

Nothing about the body changed. Only the ruler did. BMI is one of the few medical numbers where the formula is identical everywhere and the meaning is not, and most calculators never tell you which ruler they used.

Read across the dashed lines. At 23.5 the person is green in three rows and amber or orange in three others. At 26.0 they are «overweight» under the WHO but already «obese» in Korea, Japan and India. A ten-point spread in labels from a two-and-a-half-point difference in BMI.

Where the 25 and 30 came from

The WHO cut-offs — 18.5, 25, 30 — were fixed in the 1990s from mortality and morbidity curves in mostly European and North American cohorts. They are round numbers on purpose: the underlying risk curve is smooth, so any threshold is a convention, and 25 and 30 were chosen because they were memorable and roughly matched where disease risk started to climb in those populations.

The trouble surfaced when the same table was applied in Asia. Study after study found that East and South Asian adults carried more body fat, and more of it around the abdomen, at the same BMI as Europeans — typically 3 to 5 percentage points more. Type 2 diabetes, hypertension and dyslipidaemia showed up at BMIs that the international table still called normal. A Japanese or Indian population screened at 25 was being told it was fine while its diabetes rates were climbing.

The 2004 WHO compromise: 23 and 27.5

In 2004 a WHO expert consultation published in The Lancet reviewed the Asian data and reached an awkward but honest conclusion. The evidence showed that risk began earlier, but not at exactly the same BMI in every Asian population — the Chinese, Indian and Japanese curves differed from each other. So the consultation kept the international classification as the global reference and added «public health action points» at 23 and 27.5 for Asian populations: thresholds where health services should start acting, without formally redefining the words overweight and obese.

Individual countries then went their own way. Korea's Society for the Study of Obesity treats 23 as pre-obese and 25 as obesity stage 1 — a full five points below the WHO. Japan's society defines obesity from 25 but only diagnoses «obesity disease» when a weight-related disorder is present alongside it. China's working group settled on 24 and 28. India's 2009 consensus went with 23 and 25, the lowest bar of all. Singapore's health authority uses 23 and 27.5 as moderate- and high-risk lines, taking the WHO action points literally.

What the same body is called around the world

The American row deserves a note. The United States never adopted lower cut-offs, but in 2015 the American Diabetes Association lowered its diabetes-screening trigger for Asian Americans from BMI 25 to 23, citing the same evidence the WHO had weighed a decade earlier. So an Asian American can be «normal weight» on the CDC chart and «please get a fasting glucose test» from their doctor in the same visit — both correct.

Why body fat, not BMI, is the real variable

BMI never measured fat. It measures weight relative to height, and it became a health metric only because, on average, extra weight in adults is extra fat. That average hides two kinds of exception. Muscular people carry weight that is not fat and read high. And populations with a different build carry fat at a different weight and read low — which is what the Asian data revealed. Two people at BMI 25, one Dutch and one Indian, can differ by five points of body-fat percentage, and the difference is concentrated where it does the most metabolic harm: the abdomen.

That is also why every one of these bodies recommends pairing BMI with waist circumference, and why the Japanese metabolic-syndrome check-up starts with a tape measure rather than a scale. BMI is a cheap screen for a population. Waist and body fat are the questions it is standing in for.

So which ruler should you use?

The honest answer is: the one built for people who look like you, because that is the one whose thresholds were fitted to your risk. If your ancestry is East, South or Southeast Asian, read your BMI against 23 and 27.5 — or your own country's table — even if you live in London or Los Angeles; the biology did not emigrate. If you are of European descent, the international table is the one that was actually fitted to you. Either way, treat a BMI between 23 and 25 not as a verdict but as a prompt to check waist size and, past forty, fasting glucose.

And when two calculators disagree about you, notice that they are almost never disagreeing about arithmetic. They are disagreeing about which population's curve you belong to. The BMI calculator here shows the international and Asian bands on the same gauge so the disagreement is visible instead of hidden. If the next question is how many calories that body actually burns in a day, that is a separate estimate with its own error bars — the TDEE calculator covers it, including why those calculators disagree too.