What is BMI?
Body mass index (BMI) is your weight in kilograms divided by the square of your height in metres. It's a quick, equipment-free screen for whether your weight is in a healthy range for your height — the same number the World Health Organization and most public-health bodies use to compare populations.
BMI is a screening tool, not a diagnosis, and it can't tell muscle from fat. A muscular athlete and a sedentary person can share a BMI with very different body composition, which is why a high reading is a prompt to look closer — with a body-fat estimate, waist measurement, or a clinician — rather than a verdict on its own.
WHO vs. Asia-Pacific cutoffs
On the WHO international scale, a BMI under 18.5 is underweight, 18.5–24.9 is a healthy weight, 25–29.9 is overweight, and 30 or above is obesity (split into classes I, II and III at 30, 35 and 40).
The Asia-Pacific criteria shift the overweight and obesity lines down to 23 and 25. They're the standard the Korean Society for the Study of Obesity uses, because research — summarized by a WHO Expert Consultation in the Lancet in 2004 — shows that Asian populations, including Koreans, develop type 2 diabetes and cardiovascular disease at a lower BMI than European populations. The calculator shows both so you can see exactly where your number falls on each.
| Category | WHO / Asia-Pacific |
|---|---|
| Underweight | < 18.5 / < 18.5 |
| Healthy weight | 18.5–24.9 / 18.5–22.9 |
| Overweight | 25–29.9 / 23–24.9 |
| Obesity class I | 30–34.9 / 25–29.9 |
| Obesity class II | 35–39.9 / 30–34.9 |
| Obesity class III | ≥ 40 / ≥ 35 |
The same cutoffs this calculator classifies with — WHO TRS 894 (2000); KSSO 2022 guidelines for the Asia-Pacific criteria. Full citations under Formula & sources.
BMI for children and teens (2–19)
For anyone aged 2 to 19, a single fixed cutoff doesn't work, because healthy BMI changes with age and differs between boys and girls. Instead, BMI is plotted on the CDC growth reference as a percentile — how the child compares with others of the same age and sex.
Korea uses the same 85th and 95th percentile lines for children. These are screening bands; a pediatric provider is the right person to read a child's growth.
| Weight status | Percentile |
|---|---|
| Underweight | Under 5th |
| Healthy weight | 5th to under 85th |
| Overweight | 85th to under 95th |
| Obesity | 95th or higher |
| Severe obesity | 120% of the 95th, or BMI 35 |
Severe obesity starts at 120% of the 95th percentile or at a BMI of 35, whichever is lower, as the CDC defines it.
What to do with your number
If your BMI sits outside the healthy range, small sustained changes move it more reliably than crash diets: a balanced plate, enough protein, regular activity, and — if you're aiming to lose — a modest calorie deficit. Our calorie-deficit calculator estimates a sensible target, and the MyPlate Plan turns it into daily food-group amounts.
And if your BMI is in the healthy range but you carry a lot of muscle or a large waist, trust the fuller picture over the single number. BMI is the starting point, not the finish line.
Formula & sources
Body mass index
W = weight in kilograms, H = height in metres. Imperial equivalent: BMI = 703 × weight(lb) ÷ height(in)². The result is rounded to one decimal place.
BMI-for-age percentile — LMS z-score (ages 2–19)
L, M, S are the CDC BMI-for-age reference parameters (skewness, median, coefficient of variation) for the child's sex and age in months. CDC lists them per month, so a child who has completed M months is read on the M + 0.5 row; with whole years only, we read the middle of that year. The z-score converts to a percentile. Above the 95th percentile the CDC 2022 extended-percentile model is used.
Adult weight-status cutoffs (BMI, kg/m²)
WHO international vs. Asia-Pacific / KSSO. Ages 2–19 use BMI-for-age percentiles instead.
| Category | WHO · Asia-Pacific |
|---|---|
| Underweight | <18.5 · <18.5 |
| Healthy weight | 18.5–24.9 · 18.5–22.9 |
| Overweight | 25.0–29.9 · 23.0–24.9 |
| Obesity | ≥30 · ≥25 |
Sources
- World Health Organization (2000) Obesity: Preventing and Managing the Global Epidemic. WHO Technical Report Series 894 — the international BMI cut-off points (18.5, 25, 30).
- WHO Expert Consultation (2004) Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet 363(9403):157–163.
- WHO Regional Office for the Western Pacific, IASO, IOTF (2000) The Asia-Pacific perspective: redefining obesity and its treatment. Sydney: Health Communications Australia. The source of the Asian adult cutoffs: overweight from a BMI of 23, obesity from 25.
- Korean Society for the Study of Obesity (2022) Clinical Practice Guidelines for Obesity 2022 (8th ed.): overweight ≥23 and obesity ≥25 kg/m² (classes I–III at 25/30/35). J Obes Metab Syndr 31(2):100–122.
- CDC (2000; extended 2022) CDC Growth Charts and extended BMI-for-age percentiles for children and adolescents aged 2–19 years (LMS reference).