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.
Below the 5th percentile is underweight, 5th–84th is a healthy weight, 85th–94th is overweight, and 95th or above is obesity (with a severe-obesity band above 120% of the 95th percentile). Korea uses the same percentile approach for children. These are screening bands; a pediatric provider is the right place to interpret a child's growth.
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 exact age and sex; 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.
- 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).