BMI Calculator
Computes body mass index by dividing weight in kilograms by height in meters squared, applying the WHO adult classification bands at 18.5, 25 and 30 and converting imperial entries with the exact factor of 0.45359237 kg per pound. Takes height and weight in metric or imperial units and returns BMI to one decimal place, the WHO category, and the healthy weight range for the entered height.
Pick metric or imperial units, enter your height and weight, and the calculator returns your body mass index, the World Health Organization category it falls into, and the weight range that would put someone of your height inside the healthy band. Read the result as a rough position on a scale rather than a precise verdict: a BMI of 24 and a BMI of 26 fall on opposite sides of a line, but the difference in health risk between them is small and gradual. The math runs entirely in your browser and nothing you type is stored or sent anywhere.
Where the index comes from
Adolphe Quetelet (1796–1874) was a Belgian astronomer and mathematician who founded the Brussels Observatory and directed it from 1828, and who spent much of his career applying statistics to human populations. Working between roughly 1830 and 1850, he built a project he called social physics around the idea of l'homme moyen, the average man: measure a trait across a large group, and the mean value describes a kind of statistical archetype. His 1835 treatise, "Sur l'homme et le développement de ses facultés, ou Essai de physique sociale" — published in Edinburgh in 1842 as A Treatise on Man and the Development of His Faculties — set this out, and it included his observation that among fully grown adults of ordinary build weight tends to scale with the square of height rather than with height itself. That ratio, weight divided by height squared, became known as the Quetelet index.
Two things about the origin are worth holding onto. Quetelet was describing populations, not diagnosing individuals; he wanted to characterise the spread of body sizes across a group and had no interest in judging any single person. And he was not a physician. The index was a statistical convenience that happened to hold up, not a measurement grounded in physiology.
From Quetelet index to body mass index
For well over a century the ratio stayed a statistician's tool. It gained its modern name in July 1972, when the American physiologist Ancel Keys published "Indices of relative weight and obesity" in the Journal of Chronic Diseases, volume 25, pages 329 to 343, with four co-authors: Flaminio Fidanza, Martti Karvonen, Noboru Kimura and Henry Longstreet Taylor. Keys — already known for wartime ration research and the Seven Countries Study of diet and heart disease — compared several competing formulas for turning height and weight into one number and tested them against measured body fat in thousands of men drawn from several countries. The Quetelet index came out as the best simple stand-in for body fatness, and Keys proposed renaming it the body mass index. He was cautious about it, describing the measure as "as good as any other relative weight index" and recommending it for population studies rather than individual diagnosis.
WHO leaned on BMI for population monitoring through the 1980s, but the familiar bands were pinned down later. A WHO expert committee set out the adult cutoffs in its 1995 report Physical status: the use and interpretation of anthropometry, published as Technical Report Series 854, and a WHO consultation held in 1997 concluded that a coherent BMI classification was available and should be adopted internationally; its report appeared in 2000 as Technical Report Series 894. Those two documents are why 18.5, 25 and 30 now sit on screening forms worldwide.
How the number is worked out
Body mass index divides weight in kilograms by the square of height in metres:
BMI = weightkg ÷ heightm2
With the default inputs — 72.5 kg and 175 cm — the height squared is 1.75 × 1.75 = 3.0625 m², and 72.5 ÷ 3.0625 = 23.7. That lands inside the healthy band, which for 175 cm of height runs from 56.7 kg (BMI 18.5) up to 76.3 kg (BMI 24.9). To find those limits the calculator works backwards, multiplying each target BMI by the height squared: 18.5 × 3.0625 = 56.7 kg and 24.9 × 3.0625 = 76.3 kg.
In imperial mode the calculator converts pounds and inches to metric exactly before applying the same formula — one pound is 0.45359237 kg and one inch is 0.0254 m, both fixed by the 1959 International Yard and Pound Agreement. That is more precise than the rounded American shortcut of 703 × pounds ÷ inches², which folds the unit conversion into a single constant. At 160 lb and 5 ft 9 (69 inches) the shortcut gives 703 × 160 ÷ 4,761 = 23.6, and the exact conversion returns the same figure to one decimal place. The 703 is simply the number that shifts the imperial inputs onto the metric scale: 0.45359237 divided by 0.0254 squared works out to 703.07, and the familiar constant is that value rounded. It is not a separate method.
The WHO categories
| BMI | Category |
|---|---|
| below 18.5 | Underweight |
| 18.5 – 24.9 | Healthy weight |
| 25.0 – 29.9 | Overweight |
| 30.0 and above | Obesity |
WHO subdivides further — obesity classes II and III begin at 35 and 40, and severe thinness sits below 16 — but the four bands above are the ones that drive screening forms and referral criteria.
What the number misses
BMI counts every kilogram the same and cannot tell muscle from fat. Muscle is denser than fat, so a 90 kg rugby back and a 90 kg sedentary office worker who are both 1.83 m tall score an identical 26.9 despite very different body compositions and health outlooks. The index also says nothing about where fat sits. Waist circumference and waist-to-height ratio pick up abdominal fat, the type most strongly tied to metabolic risk, and the NHS advises keeping your waist size to less than half your height.
Age shifts the picture too. A 2014 meta-analysis in the American Journal of Clinical Nutrition pooled 32 studies covering close to 198,000 community-dwelling adults aged 65 and over, followed for an average of 12 years. Taking a BMI of 23.0 to 23.9 as the reference, mortality risk was 12 per cent higher at 21.0 to 21.9 and 19 per cent higher at 20.0 to 20.9, while no increase showed up at the top of the scale until BMI passed about 33. Being overweight by the adult bands carried no measurable mortality penalty in that age group, so modest extra weight after 65 does not read the same way it does at 35. None of this makes the number worthless — across whole populations it tracks obesity-related risk well, which is exactly what Quetelet and Keys built it for — but for one person it is a starting point rather than a conclusion.
Cutoffs are not universal
The 18.5, 25 and 30 thresholds were derived largely from European-ancestry populations, and the relationship between BMI and body fat varies with ancestry. A WHO expert consultation on Asian populations, reported in The Lancet in January 2004, found that the BMI at which observed risk begins to climb ranges from 22 to 25 across different Asian groups and that the high-risk point ranges from 26 to 31. It declined to redefine the cutoffs for each population, kept the WHO numbers as the international classification, and instead flagged 23.0, 27.5, 32.5 and 37.5 as public health action points along the BMI continuum. UK practice follows the first two of those: NHS and NICE guidance treats 23 as increased risk and 27.5 as high risk for adults with South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family backgrounds. The US CDC, Health Canada and Australia's national guidelines keep the standard WHO bands for the general adult population. None of the adult cutoffs apply to children or teenagers, who are assessed against age- and sex-specific growth-chart percentiles instead — in the US the CDC flags the 85th percentile as overweight and the 95th as obesity for ages 2 to 19.
BMI is a screening measure, not a diagnosis, and a figure outside the healthy band is a prompt to talk to a clinician rather than a verdict. See the site disclaimer.
Frequently asked questions
What is a healthy BMI for adults?
The WHO healthy range is 18.5 to 24.9 for adults of all ages and both sexes. For someone 175 cm tall that translates to 56.7 to 76.3 kg, and at 5 ft 9 it is roughly 125 to 169 lb. Values just above 25 carry only modestly higher risk, so the gradient matters more than the exact cutoff.
How do I calculate BMI with pounds and inches?
The standard shortcut is 703 times your weight in pounds divided by your height in inches squared. At 160 lb and 5 ft 9 (69 inches) that is 703 × 160 ÷ 4,761 = 23.6. This calculator converts to kilograms and meters exactly instead, which changes the result by less than 0.01.
Is a BMI of 27 overweight?
Under the WHO bands, yes — 25.0 to 29.9 is the overweight category. It is the middle of that band rather than the edge of obesity, and for adults over 65 several large cohort studies have found the lowest mortality near a BMI of 27. Waist size and fitness deserve a look alongside the number.
Why is my BMI high if I am muscular?
BMI cannot tell muscle from fat, and muscle is denser, so lean athletic builds routinely score 26 to 30 while carrying little body fat. Body-fat percentage or waist-to-height ratio gives a fairer picture in that case. Keeping your waist under half your height is a widely used rule of thumb.
Does BMI work for children and teenagers?
Not with adult cutoffs. Anyone under 20 is assessed against age- and sex-specific growth-chart percentiles — in the US, the CDC flags the 85th percentile as overweight and the 95th as obesity. A BMI of 21 can be perfectly typical for a 16-year-old and concerning for a 9-year-old.