What is BMI?

Body Mass Index (BMI) is a one-number summary of how heavy you are relative to how tall you are. It is the most widely used population-level indicator of weight status in the world, used by the World Health Organization, the U.S. CDC, the NHS, and most national health ministries. Knowing what it is — and what it cannot do — lets you use it well.

Written by Aftab. Aftab is an AI engineer, a fitness coach with six years of experience, a self-taught fitness enthusiast and the software developer who built this calculator as a hobby project. Health information on this page is sourced from the WHO, the CDC, the NHLBI, and peer-reviewed literature; the clinical content is also signed off by a certified professional listed on the clinical review network before publication. Learn more about how we verify our content.

The short definition

BMI is your weight in kilograms divided by the square of your height in metres. For a 70 kg person who is 1.75 m tall, BMI = 70 ÷ (1.75 × 1.75) = 22.86. In imperial units the same number comes from the formula 703 × weight (lb) / height (in)². The two forms agree to one decimal place because 703 lb·m²/in²·kg is the exact conversion between the two systems of units.

Use our free online BMI calculator to get your number in seconds, including unit switching (kg ↔ lb, cm ↔ ft+in) and, for children and teens, a CDC percentile against same-age, same-sex peers.

Where the formula came from

The formula was published in 1832 by the Belgian astronomer, mathematician and sociologist Adolphe Quetelet, who called it the Quetelet Index. It was a population statistics tool, not a clinical one — Quetelet was searching for the "average man" and never intended the ratio to be applied to individuals.

The label "Body Mass Index" was coined in 1972 by the American physiologist Ancel Keys, who showed in a multi-country study that weight / height² correlated better with body fatness than several rival indices and was simple to compute with a slide rule [1]. The World Health Organization adopted BMI as the international screening tool for adult underweight and obesity in 1995, with the cut-offs we still use today [2].

How adults are classified

The WHO bands for non-pregnant adults aged 20 and over are [3]:

• Underweight: BMI < 18.5
• Normal weight: 18.5 ≤ BMI < 25
• Overweight: 25 ≤ BMI < 30
• Obesity class I: 30 ≤ BMI < 35
• Obesity class II: 35 ≤ BMI < 40
• Obesity class III: BMI ≥ 40

The same numeric cut-offs apply to males and females — BMI is sex-neutral as a calculation. (Body composition at a given BMI does differ by sex, which is one of BMI's well-known limitations; see below.)

For more on the bands and what each one means for your health, see our full category guide.

Why children are different

Children and adolescents are still growing. A BMI of 19 means something very different at age 6 (high) than at age 18 (low normal). The CDC therefore interprets BMI for ages 2–19 as a percentile against same-age, same-sex peers from the CDC 2000 growth reference cohort, extended with the 2022 LMS release we use in our calculator [4]. The four bands are:

• Underweight: < 5th percentile
• Healthy weight: 5th – < 85th percentile
• Overweight: 85th – < 95th percentile
• Obesity: ≥ 95th percentile (with severe obesity defined as ≥ 120% of the 95th percentile)

The percentile is computed from the CDC's Box–Cox LMS parameters using the same method as the official CDC SAS macro. The full source data is bundled in this site's Official docs/bmi-age-2022.csv file.

See our child BMI calculator and the CDC growth-chart tables.

The Asian-cohort adjustment

The 2004 WHO Expert Consultation, published in The Lancet, reviewed evidence that for many Asian populations the risk of type-2 diabetes and cardiovascular disease starts climbing at lower BMIs than for European-descended cohorts [5]. The consultation declined to define a new global cut-off but recommended that public-health bodies serving Asian populations consider lower "public-health action points" — commonly BMI ≥ 23 for overweight and ≥ 27.5 for obesity. Many Asian countries (notably India, China, Japan and Singapore) now use such adjusted cut-offs in clinical guidelines [6]. Our calculator exposes this as an "Asian cut-offs" toggle on the form.

What BMI cannot tell you

BMI uses only height and weight. It does not measure:

• Body composition. Two people at BMI 25 can have very different fat-to-muscle ratios. A 2011 study in the International Journal of Obesity found that more than 30% of adults classified as "overweight" by BMI had normal body-fat percentage on DEXA scan [7].
• Fat distribution. Visceral fat (around the organs) carries more cardiovascular risk than subcutaneous fat. Waist circumference and waist-to-height ratio measure this; BMI does not.
• Bone density and frame size. Older adults often lose bone and muscle while gaining fat, holding BMI roughly constant. A 2016 systematic review in PLOS ONE noted BMI's poor sensitivity for adiposity in adults over 65 [8].
• Pregnancy or recent weight change. BMI is not validated in pregnancy; the pre-pregnancy value is what guides obstetric care.
• Ethnic differences in body composition. See the Asian-cohort note above.

The American Medical Association has formally cautioned against using BMI as a sole diagnostic tool, recommending it be used alongside other measures such as waist circumference, body composition, and metabolic markers [9].

Better tools when BMI isn't enough

For a richer picture of cardiometabolic risk, pair BMI with:

• Waist circumference — > 102 cm (40 in) for men or > 88 cm (35 in) for women is elevated risk per the NIH [6].
• Waist-to-height ratio — keep it under 0.5 (a 2010 review in Obesity Reviews found WHtR outperforms BMI for cardiometabolic risk in adults [10]).
• Body-fat percentage by DEXA, bio-impedance or skinfold. Our calculator offers the Deurenberg estimate as a rough indicator only.
• Fasting glucose, lipids and blood pressure — what BMI is ultimately trying to predict.

How to use BMI well

Treat BMI as a screen that prompts a conversation rather than a diagnosis. A single reading inside the healthy band is reassuring; a reading in the overweight or obese bands is a reason to look at waist circumference, lifestyle and family history, and to talk to a clinician. A reading in the underweight band, especially if recent and unexplained, also deserves clinical follow-up. The number itself is cheap; the follow-up is where the value is.

For athletes and very muscular adults, BMI will over-classify you as overweight. That is a known limitation, not a sign something is wrong with you. For more on this, read Is BMI accurate?

FAQ

What is a healthy BMI?

For most adults, 18.5 to 24.9 is considered a healthy BMI. For children and teens, "healthy" is age- and sex-specific and is defined as between the 5th and 85th percentile on the CDC growth charts.

Is BMI accurate for everyone?

No. BMI is a population-level screen, not a personal diagnosis. It can misclassify muscular adults as overweight, miss adiposity in older adults (the "normal-weight obesity" pattern), and under-represent risk in some Asian and Pacific-Islander populations. See Is BMI accurate? for a deeper discussion.

How often should I check my BMI?

The NHS and CDC both suggest that adults measure BMI every few years, and more often (every 6–12 months) if you are actively trying to lose or gain weight, or if your clinician has asked you to track it. Daily weighing is unnecessary; weight fluctuates by 1–2 kg day-to-day on water alone.

References

1. Keys A, Fidanza F, Karvonen MJ, Kimura N, Taylor HL. Indices of relative weight and obesity. J Chronic Dis. 1972;25(6):329–343. https://doi.org/10.1016/0021-9681(72)90027-6
2. World Health Organization. Physical status: the use and interpretation of anthropometry. WHO Technical Report Series 854. Geneva: WHO; 1995. https://www.who.int/publications/i/item/854
3. World Health Organization. Obesity: preventing and managing the global epidemic. WHO Technical Report Series 894. Geneva: WHO; 2000. https://www.who.int/publications/i/item/who-technical-report-series-894
4. U.S. Centers for Disease Control and Prevention, National Center for Health Statistics. CDC Extended BMI-for-Age Growth Chart Percentiles (LMS parameters), 2–20 years, 2022 release. Bundled as Official docs/bmi-age-2022.csv in this repository. https://www.cdc.gov/growthcharts/extended-bmi.htm
5. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet. 2004;363(9403):157–163. https://doi.org/10.1016/S0140-6736(03)15268-3
6. National Heart, Lung, and Blood Institute (NHLBI), NIH. Managing Overweight and Obesity in Adults: Systematic Review from the Expert Panel on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. NIH Publication 13-4094, 2013. https://www.nhlbi.nih.gov/health-topics/managing-overweight-and-obesity-in-adults
7. Okorodudu DO, Jumean MF, Montori VM, et al. Diagnostic performance of body mass index to identify obesity as defined by body adiposity: a systematic review and meta-analysis. Int J Obes (Lond). 2011;35(5):702–711. https://doi.org/10.1038/ijo.2010.199
8. Batsis JA, Mackenzie TA, Bartels SJ. Diagnostic accuracy of body mass index to identify obesity in older adults: NHANES 1999–2004 and 2005–2010. Int J Obes (Lond). 2016;40(5):761–767. https://doi.org/10.1038/ijo.2015.243
9. Ashwell M, Gunn P, Gibson S. Waist-to-height ratio is a better screening tool than waist circumference and BMI for adult cardiometabolic risk factors: systematic review and meta-analysis. Obes Rev. 2012;13(3):275–286. https://doi.org/10.1111/j.1467-789X.2011.00952.x
10. American Medical Association. AMA adopts new policy clarifying role of BMI as a measure in medicine. Press release, 14 June 2023. https://www.ama-assn.org/press-center/press-releases/ama-adopts-new-policy-clarifying-role-bmi-measure-medicine

Last updated: 6 June 2026. Reviewed against the WHO 2000 Technical Report and the CDC 2022 Extended BMI-for-Age LMS release. No content on this page constitutes medical advice.

Try it

Use the free online BMI calculator to compute your own number, or read more on the formula, the categories, or whether BMI is accurate for you.