What BMI is and how it is calculated
Body mass index is weight in kilograms divided by height in metres squared. The ratio was devised by the Belgian statistician Adolphe Quetelet in the 1830s to describe populations, and only became a screening number for individuals in the 1970s. That history explains both its usefulness — one number, two easy measurements — and its limits.
A metric example: 172 cm and 68 kg gives 1.72² = 2.9584, and 68 ÷ 2.9584 = 22.99, so BMI 23.0. An imperial example: 5 ft 9 in and 165 lb converts to 1.7526 m and 74.84 kg, giving 24.4.
Imperial input is converted to metres and kilograms and then run through the same formula, rather than using the 703 × lb ÷ in² shortcut. The shortcut is a rounded approximation, and mixing the two paths is why the same person can get slightly different numbers on different sites.
The WHO categories and what your number means
The WHO international classification for adults splits the scale into eight bands, each open at the top so a BMI of exactly 25.0 belongs to the band above:
- under 16.0 severe thinness · 16.0–17.0 moderate thinness · 17.0–18.5 mild thinness
- 18.5–25.0 normal range
- 25.0–30.0 overweight (pre-obese) · 30.0–35.0 obese class I · 35.0–40.0 class II · 40.0 and over class III
The category is decided from the unrounded value, so a BMI of 24.96 is shown as 25.0 for readability but still classified as normal. That is deliberate: rounding first would move people across a boundary they have not actually crossed.
Why the Asian-Pacific thresholds are lower
A WHO expert consultation published in The Lancet in 2004 found that the risk of type 2 diabetes and cardiovascular disease rises at a lower BMI in many Asian populations, largely because body fat is higher at the same BMI. The Asian-Pacific guidance that followed treats 23.0 and above as overweight or at increased risk, and 25.0 and above as obese.
Korean clinical practice follows the same cut-offs: the Korean Society for the Study of Obesity classifies 23.0–24.9 as pre-obesity and 25.0 and above as obesity, with further stages at 30 and 35. That is why a number that reads “normal” on an American site can read “overweight” at a Korean health check.
Switching the standard on this page changes the label, the coloured scale and the healthy weight range together, so you can see what the same body weight means under each set of criteria rather than converting in your head.
Your healthy weight range, BMI Prime and the Ponderal Index
A BMI on its own is hard to act on, so the page converts the normal band back into kilograms for your height. At 172 cm the WHO normal range is 18.5 × 2.9584 = 54.7 kg to 25 × 2.9584 = 74.0 kg; under the Asian-Pacific standard the upper edge drops to 23 × 2.9584 = 68.0 kg.
If you are outside the range the tool also shows the distance to the nearest edge in kilograms and pounds — the number that turns an abstract score into a goal. Every band, not just the normal one, is listed as a weight range, so you can see exactly where the next boundary sits.
Two derived figures sit beside the result. BMI Prime is your BMI divided by 25, so 1.00 is exactly the top of the WHO normal range and 1.12 means 12% above it. The Ponderal Index divides weight by height cubed instead of squared, which holds up better at the extremes of height where BMI tends to overstate tall people and understate short ones.
What BMI does not measure
BMI counts kilograms, not what they are made of. Muscle is denser than fat, so athletes and heavily trained people are routinely pushed into the overweight band while carrying little fat, and older adults who have lost muscle can sit in the normal band with a high body fat percentage.
It also ignores where fat sits, and abdominal fat carries most of the metabolic risk. That is why health checks pair BMI with waist circumference — the Korean Society for the Study of Obesity sets the abdominal obesity threshold at 90 cm for men and 85 cm for women — or with the waist-to-height ratio, which many guidelines now suggest keeping under 0.5.
The adult bands do not apply to children and teenagers, who are assessed against age-and-sex percentile charts (in Korea, the 2017 growth charts for children and adolescents), and they are not meant for pregnancy. BMI is a screening number that suggests a conversation with a clinician; it is never a diagnosis.
Using BMI sensibly over time
A single reading says less than a trend. Weigh yourself under the same conditions — same time of day, similar clothing, after the same kind of day — and compare over weeks, not hours; normal daily fluctuation of one to two kilograms from food, salt and hydration is larger than most real weekly change.
Where weight loss is clinically indicated, guidance usually starts with 5–10% of current body weight rather than a target BMI, because that is the range where blood pressure, blood sugar and lipids measurably improve. For an 88 kg adult that is 4.4–8.8 kg — a goal the healthy-range figures on this page make easy to track.