Body Mass Index_
Estimate your BMI from height and weight, then read a plain-language, sourced explanation of what the number means — and, just as importantly, what it can't tell you.
BMI is a screening estimate, not a diagnosis or a measure of health or worth. See the limitations below.
What is BMI?
Body mass index (BMI) is a single number that relates a person's weight to their height. It is calculated by dividing weight in kilograms by the square of height in metres, giving units of kg/m².1 Because it needs only a scale and a tape measure, it is quick, inexpensive, non-invasive, and easy to collect during routine care — which is why it became the most widely used population screen for weight status.2
Crucially, BMI does not measure body fat directly. It is a proxy: at a population level it correlates moderately to strongly with body-fat measures, but on its own it cannot tell whether an individual's weight comes from fat, muscle, or bone.2,4 Health authorities therefore describe BMI as a screening tool that flags whether further assessment may be useful — not a diagnostic test.2
How to calculate it
There are two equivalent formulas depending on the units you use.
Metric
Height is in metres, so 170 cm becomes 1.70 m.1
Imperial (US customary)
The factor of 703 converts pounds-per-square-inch onto the metric kg/m² scale, so both formulas land on the same number.3
BMI categories (adults)
For adults aged 20 and over, the World Health Organization and CDC use the same cut-offs, and they apply identically to men and women of all body types.1,2
| Category | BMI range (kg/m²) |
|---|---|
| Underweight | Below 18.5 |
| Healthy weight | 18.5 – 24.9 |
| Overweight | 25.0 – 29.9 |
| Obesity | 30.0 and above |
Obesity is sometimes sub-divided into class I (30.0–34.9), class II (35.0–39.9), and class III (40.0 and above).1 Children and teens are assessed differently — their BMI is compared against age- and sex-specific percentiles rather than these fixed numbers.3
A short history
The formula was devised in the 1830s by the Belgian mathematician and statistician Adolphe Quetelet, who was studying the statistical traits of the "average man" — not diagnosing individuals. For over a century it was known as the Quetelet Index.5
In a 1972 paper in the Journal of Chronic Diseases, the American physiologist Ancel Keys and colleagues tested several height–weight formulas against body-fat measurements and concluded Quetelet's ratio was the best simple index for population studies. That paper gave the measure its modern name: the body mass index.6 The WHO and US National Institutes of Health later standardised today's category cut-offs in the 1990s.4
Important limitations
BMI is useful at population scale, but it has real, well-documented blind spots for individuals:
It can't separate fat from muscle or bone. Because it uses only total weight, a very muscular athlete can register as "overweight" despite low body fat, and the number says nothing about where fat sits on the body.2,4
It varies with age, sex, and ethnicity. At the same BMI, older adults tend to carry more body fat than younger adults, and athletes less.2 The original cut-offs were derived largely from non-Hispanic white populations; some groups, including people of Asian ancestry, can face weight-related health risks at lower BMI values, which is why several guidelines use lower thresholds for them.8
Major medical bodies urge caution. In 2023 the American Medical Association adopted policy describing BMI as an "imperfect" clinical measure that should be used alongside other indicators — body composition, waist circumference, family history — rather than on its own.7,8
References
- World Health Organization. Obesity and overweight (fact sheet). who.int
- Centers for Disease Control and Prevention. About Body Mass Index (BMI). cdc.gov/bmi/about
- Centers for Disease Control and Prevention. BMI Frequently Asked Questions (×703 imperial formula; child/teen percentiles). cdc.gov/bmi/faq
- Weir CB, Jan A. Physiology, Body Mass Index. StatPearls, NCBI Bookshelf. ncbi.nlm.nih.gov/books/NBK535456
- Eknoyan G. Adolphe Quetelet (1796–1874) — the average man and indices of obesity. Nephrol Dial Transplant. 2008;23(1):47–51. academic.oup.com
- Blackburn H, Jacobs D. Commentary: origins and evolution of body mass index — with the reprint of Keys et al. (1972). Int J Epidemiol. 2014;43(3):665–669. academic.oup.com
- American Medical Association. AMA: Use of BMI alone is an imperfect clinical measure. 2023. ama-assn.org
- AMA Council on Science and Public Health. Report on the use of BMI (A-23) — race/ethnicity differences; higher body fat at equivalent BMI in Asian populations. 2023. ama-assn.org (PDF)