How BMI is calculated

The formula

BMI = weight (kg) ÷ height (m)²
BMI = 703 × weight (lb) ÷ height (in)²

For 165 lb at 5 ft 9 in (69 inches): 703 × 165 ÷ 69² = 115,995 ÷ 4,761 = 24.4. In metric, 75 kg at 1.75 m: 75 ÷ 3.0625 = 24.5.

The 703 is a unit conversion: 0.45359237 kg per pound ÷ (0.0254 m per inch)² = 703.07. This site converts with the exact legal definitions — 1 lb = 0.45359237 kg and 1 in = 2.54 cm — and then uses the metric formula, so a US entry and a metric entry of the same body always give the same BMI.

Rounding and categories

BMI is reported to one decimal, and the category is read from that rounded value. This is the CDC's convention, and it matters at the edges: a BMI of 24.96 is shown as 25.0 and is overweight, never "healthy".

The same cut-offs apply to men and women aged 20 and over.

The healthy weight range

A healthy weight range is every weight whose rounded BMI is 18.5 to 24.9. At 5 ft 9 in: 124 lb is a BMI of 18.31 (18.3, underweight); 125 lb is 18.46, which rounds to 18.5; 168 lb is 24.81 (24.8); and 169 lb is 24.96, which rounds to 25.0. So the range is 125–168 lb — exactly the figures the CDC publishes for that height. In kilograms the range is worked out the same way, to a tenth of a kilogram.

Asian and national cut-offs

People of Asian background tend to have more body fat, and more of it around the abdomen, at the same BMI, and their risk of type 2 diabetes and heart disease rises at a lower BMI. A 2004 WHO expert consultation proposed action points at 23, 27.5, 32.5 and 37.5; NICE in the UK uses 23 for overweight and 27.5 for obesity for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African and African-Caribbean family background. Several countries set their own lines:

The Asian BMI calculator classifies a BMI under all of them at once.

Children and teens (2 to 19)

A child's healthy BMI changes with age and differs between boys and girls, so it is compared with a reference group — the CDC 2000 growth charts — and given as a percentile:

The LMS method. For each sex and month of age the CDC publishes three numbers: L (skewness), M (the median) and S (spread). A BMI becomes a z-score with z = ((BMI ÷ M)^L − 1) ÷ (L × S), and the percentile is the normal distribution's value at z. This site uses CDC's reference values for every month from 24 to 240, interpolated between months as CDC's own software does, and places a child of whole months at the middle of the month.

The extended method above the 95th percentile. The 2000 charts compress the upper tail: almost every child with severe obesity comes out at the 99th percentile. Since 2022 the CDC has used a different model there: percentile = 90 + 10 × Φ((BMI − P95) ÷ σ), where P95 is the BMI at the 95th percentile and σ is a smooth function of age that differs by sex. This site follows it. CDC's check value: a boy of 50.5 months with a BMI of 22.6 is at the 99.7th percentile, 127% of the 95th.

Severity above the 95th percentile follows the American Academy of Pediatrics' 2023 guideline: class 2 severe obesity from 120% of the 95th percentile, class 3 from 140%.

Older adults and pregnancy

After 65, the categories are the same but their meaning shifts. A 2014 meta-analysis of nearly 200,000 people aged 65 and over found no extra risk of death in the overweight range, and higher risk below a BMI of 23. For malnutrition screening, the Global Leadership Initiative on Malnutrition (GLIM) treats a BMI under 20 before age 70, or under 22 from 70, as low (18.5 and 20 for Asian adults), and losing more than 5% of body weight in six months as a warning sign.

In pregnancy BMI is not used, because the weight of the baby, placenta and fluid is not body fat. The pre-pregnancy BMI sets the recommended weight gain, following the Institute of Medicine's 2009 guidelines.

BMI Prime and the new BMI

BMI Prime is BMI ÷ 25, the upper limit of a healthy BMI. Under 1.00 is below the overweight line; 1.20 means 20% above it.

The new BMI, proposed by Oxford mathematician Nick Trefethen in 2013, is 1.3 × weight (kg) ÷ height (m)^2.5. Body size does not scale with the square of height: ordinary BMI makes tall people look heavier and short people lighter than they are. The 2.5 power corrects much of that, and the 1.3 factor makes the two agree for a person of about 1.69 m (5 ft 6.5 in). For 165 lb at 5 ft 9 in the new BMI is 23.9, against an ordinary BMI of 24.4. It is not used in guidelines.

The ponderal index (Rohrer's index) is weight ÷ height³, in kg/m³. It is mostly used for newborns.

Where BMI falls short

• Muscle and fat weigh the same to BMI. Athletes and people who lift weights can be "overweight" with little fat; people with little muscle can carry too much fat at a "healthy" BMI.
• It cannot see where fat is. Fat around the abdomen and organs is the riskier kind. A waist under half your height is the simplest check (waist-to-height ratio).
• The same BMI means different things by sex, age and ethnic background — more body fat in women than men, in older than younger adults, and higher risk at the same BMI in people of Asian background.

In 2023 the American Medical Association advised that BMI should not be used on its own in clinical care. In 2025 a commission of 58 experts in The Lancet Diabetes & Endocrinology recommended confirming excess body fat in anyone with a high BMI — with a waist measurement, a waist-to-hip or waist-to-height ratio, or a direct body-fat measurement — before diagnosing obesity, except at a BMI above 40, where excess fat can be assumed. It also separated preclinical obesity (excess fat with normal organ function) from clinical obesity (excess fat that is already causing illness or limiting daily life).

The other formulas on this site

• Body fat: the US Navy tape method (Hodgdon and Beckett, 1984), relative fat mass (Woolcott and Bergman, 2018) and the BMI-based Deurenberg equation (1991) — body fat calculator.
• Waist: waist-to-height ratio with NICE's bands, waist-to-hip ratio with the WHO's 0.90 and 0.85 lines, and the body roundness index (Thomas, 2013) with the bands of a 2024 US study.
• Ideal weight: the Devine, Robinson, Miller and Hamwi formulas — ideal weight calculator.
• Energy: Mifflin–St Jeor, the revised Harris–Benedict and Katch–McArdle equations for BMR, times the standard activity factors — BMR and calorie calculators.

Where BMI comes from

In the 1830s the Belgian astronomer and statistician Adolphe Quetelet observed that adult weight rises roughly with the square of height. In 1972 the physiologist Ancel Keys compared several height–weight indices against body fat measured in thousands of men, found Quetelet's ratio the best of them, and named it the body mass index. The WHO's current adult categories were set in the 1990s, and the US National Institutes of Health adopted the same cut-offs in 1998.

Sources

1. Obesity and overweight (fact sheet) — World Health Organization
2. Adult BMI Categories — Centers for Disease Control and Prevention
3. Body Mass Index (BMI) — Centers for Disease Control and Prevention
4. Calculate Your Body Mass Index — National Heart, Lung, and Blood Institute (NIH)
5. Child and Teen BMI Categories — Centers for Disease Control and Prevention
6. CDC Extended BMI-for-age Growth Charts (2022) — Centers for Disease Control and Prevention
7. CDCAnthro: reference data and code for the CDC growth charts — CDC Division of Nutrition, Physical Activity, and Obesity
8. Hampl SE et al. Clinical Practice Guideline for the Evaluation and Treatment of Children and Adolescents With Obesity. Pediatrics 2023;151(2):e2022060640 — American Academy of Pediatrics
9. WHO Expert Consultation. Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies. Lancet 2004;363:157–163 — The Lancet
10. Overweight and obesity management (NG246), 2025 — National Institute for Health and Care Excellence (UK)
11. Zhou BF; Cooperative Meta-Analysis Group of the Working Group on Obesity in China. Predictive values of body mass index and waist circumference for risk factors of certain related diseases in Chinese adults. Biomed Environ Sci 2002;15:83–96 — Biomedical and Environmental Sciences
12. Examination Committee of Criteria for “Obesity Disease” in Japan. New criteria for “obesity disease” in Japan. Circ J 2002;66:987–992 — Japan Society for the Study of Obesity
13. Misra A et al. Consensus statement for diagnosis of obesity, abdominal obesity and the metabolic syndrome for Asian Indians. J Assoc Physicians India 2009;57:163–170 — Journal of the Association of Physicians of India
14. Kim BY et al. 2020 Korean Society for the Study of Obesity Guidelines for the Management of Obesity in Korea. J Obes Metab Syndr 2021;30:81–92 — Korean Society for the Study of Obesity
15. Cederholm T et al. GLIM criteria for the diagnosis of malnutrition. Clin Nutr 2019;38:1–9 — Clinical Nutrition
16. Winter JE et al. BMI and all-cause mortality in older adults: a meta-analysis. Am J Clin Nutr 2014;99:875–890 — American Journal of Clinical Nutrition
17. Rubino F et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol 2025;13:221–262 — The Lancet Diabetes & Endocrinology
18. AMA: Use of BMI alone is an imperfect clinical measure (2023) — American Medical Association
19. Keys A et al. Indices of relative weight and obesity. J Chronic Dis 1972;25:329–343 — Journal of Chronic Diseases
20. Trefethen N. A new BMI — University of Oxford
21. Ashwell M, Gibson S. Waist-to-height ratio as an indicator of “early health risk”. BMJ Open 2016;6:e010159 — BMJ Open
22. Waist circumference and waist–hip ratio: report of a WHO expert consultation — World Health Organization

For general information only — not a diagnosis or a substitute for advice from a doctor.