# 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](#bmi-ref-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](#bmi-ref-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](#bmi-ref-2),[4](#bmi-ref-4) Health authorities therefore describe BMI as a *screening* tool that flags whether further assessment may be useful — not a diagnostic test.[2](#bmi-ref-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](#bmi-ref-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-ref-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](#bmi-ref-1),[2](#bmi-ref-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](#bmi-ref-1) Children and teens are assessed differently — their BMI is compared against age- and sex-specific percentiles rather than these fixed numbers.[3](#bmi-ref-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](#bmi-ref-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](#bmi-ref-6) The WHO and US National Institutes of Health later standardised today's category cut-offs in the 1990s.[4](#bmi-ref-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](#bmi-ref-2),[4](#bmi-ref-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](#bmi-ref-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](#bmi-ref-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](#bmi-ref-7),[8](#bmi-ref-8) ## References 1. World Health Organization. *Obesity and overweight* (fact sheet). [who.int](https://www.who.int/news-room/fact-sheets/detail/obesity-and-overweight) 2. Centers for Disease Control and Prevention. *About Body Mass Index (BMI)*. [cdc.gov/bmi/about](https://www.cdc.gov/bmi/about/index.html) 3. Centers for Disease Control and Prevention. *BMI Frequently Asked Questions* (×703 imperial formula; child/teen percentiles). [cdc.gov/bmi/faq](https://www.cdc.gov/bmi/faq/index.html) 4. Weir CB, Jan A. *Physiology, Body Mass Index*. StatPearls, NCBI Bookshelf. [ncbi.nlm.nih.gov/books/NBK535456](https://www.ncbi.nlm.nih.gov/books/NBK535456/) 5. Eknoyan G. Adolphe Quetelet (1796–1874) — the average man and indices of obesity. *Nephrol Dial Transplant*. 2008;23(1):47–51. [academic.oup.com](https://academic.oup.com/ndt/article/23/1/47/1923176) 6. 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](https://academic.oup.com/ije/article/43/3/665/2949550) 7. American Medical Association. *AMA: Use of BMI alone is an imperfect clinical measure*. 2023. [ama-assn.org](https://www.ama-assn.org/public-health/chronic-diseases/ama-use-bmi-alone-imperfect-clinical-measure) 8. 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)](https://www.ama-assn.org/system/files/a23-csaph07.pdf) ## Accessibility ### Screen reader support --- *Source: [ocxly.com/bmi.html](https://ocxly.com/bmi.html) — OCXLY, free 100% client-side privacy-first tools. A body mass index (BMI) calculator with a plain-language, sourced explanation of what BMI is, how it is calculated, its categories, history, and limitations.*