Body Composition
BMI Calculator
Computes Body Mass Index (kg/m^2) from height and weight and classifies it against WHO adult weight categories.
BMI at a glance
Computes Body Mass Index (kg/m^2) from height and weight and classifies it against WHO adult weight categories.
Rather than repeating a single reference figure, the BMI below personalizes it: your own measurements feed the same published equation the reference ranges are built from.
Nothing here is a black box - the script embedded in this page applies the published equation shown in the code with the inputs you enter, and returns the same precision you would get working it out manually. If you log meals, this is the companion number: it uses the same units and daily framing as the diary, which makes comparing your intake against the target a one-glance check rather than a conversion exercise.
About BMI Calculator
The Body Mass Index (BMI) is the most widely used anthropometric screening tool in clinical and public health practice. It expresses the ratio of body weight to the square of height (kg/m^2) and was popularized in its modern form by the Belgian astronomer Adolphe Quetelet in 1832, which is why it was originally called the Quetelet Index. The term Body Mass Index was coined by Ancel Keys and colleagues in a 1972 paper in the Journal of Chronic Diseases, where they evaluated several weight-for-height indices and concluded that Quetelet's was the best correlate of body fatness across populations. Today BMI underpins the World Health Organization classification of underweight, normal weight, overweight, and obesity that is used in nearly every adult preventive health visit.How the formula works
The formula is deliberately simple: BMI = weight (kg) / height (m)^2. A 75 kg adult who is 1.75 m tall has a BMI of 75 / 3.0625 = 24.5, which falls at the upper end of the normal range. The squaring of height creates an index that is, in theory, independent of stature, although in practice taller adults tend to have slightly higher BMI values than shorter adults at the same body composition. For imperial inputs the conversion is BMI = 703 x weight (lb) / height (in)^2.Interpretation and WHO categories
The WHO classifies adult BMI into six categories. Underweight is defined as BMI below 18.5 kg/m^2, normal weight as 18.5-24.9, overweight as 25.0-29.9, and obesity class I, II, and III as 30.0-34.9, 35.0-39.9, and 40.0 or greater respectively. These thresholds were derived from observational data linking BMI to all-cause mortality and cardiovascular risk, with the nadir of the mortality curve falling in the 22.5-24.9 range in most large pooled analyses such as the Prospective Studies Collaboration (Lancet 2009). Some Asian populations use lower cut points (23 for overweight, 27.5 for obesity) because cardiometabolic risk rises at lower BMI in these groups, a position endorsed by the WHO Western Pacific Region and the International Diabetes Federation.Evidence and predictive value
BMI is highly correlated with body fat percentage measured by densitometry, with correlation coefficients typically in the 0.7-0.9 range in adults, though it does not distinguish fat mass from lean mass. The Global BMI Mortality Collaboration (Lancet 2016) pooled 239 prospective studies and nearly 4 million participants and confirmed a J-shaped association between BMI and all-cause mortality, with the lowest risk in the 18.5-24.9 range. Each 5 kg/m^2 increment above 25 kg/m^2 was associated with roughly 31 percent higher all-cause mortality. Below 18.5, mortality also rises, partly reflecting illness-related weight loss.Limitations
BMI's chief limitation is that it cannot distinguish fat from lean tissue and does not account for fat distribution. A 100 kg bodybuilder at 180 cm has a BMI of 30.9 yet may carry 10 percent body fat; conversely, an elderly person with a normal BMI may have sarcopenic obesity, with low muscle and high visceral fat. BMI also systematically overestimates adiposity in tall individuals and underestimates it in short individuals. For these reasons BMI should be paired with a waist circumference measurement or a direct body composition method such as skinfolds, bioelectrical impedance, or DEXA when individual risk stratification is required. The American Medical Association's 2023 council report explicitly acknowledged these limitations and called for BMI to be used in conjunction with other measures rather than as a sole indicator of health.How to use the result
BMI is best used as a population screening tool and a personal tracking metric. A result in the normal range suggests average cardiometabolic risk, but it should be interpreted alongside waist circumference, blood pressure, lipids, glucose, family history, and fitness level. Trends in BMI over time are often more informative than a single value, particularly when combined with strength and waist measurements to ensure that weight loss reflects fat rather than muscle.
Editorial and formula review
Formula sources, population limits, and references for this body composition calculator are documented in our public methodology. Read the methodology · Medical disclaimer
Frequently asked questions
BMI can overestimate adiposity in muscular individuals because muscle is denser than fat. A lean bodybuilder may register as 'overweight' or 'obese' on BMI despite low body fat. For athletes, body composition methods such as skinfolds, DEXA, or the FFMI calculator are more informative.
Several Asian populations develop type 2 diabetes and cardiovascular disease at lower BMI values than white populations, partly due to visceral fat distribution. The WHO Western Pacific Region recommends overweight at BMI >= 23 and obesity at BMI >= 27.5 for these populations.
In older adults BMI loses some accuracy because of age-related muscle loss (sarcopenia). An older person with a 'normal' BMI may still have excess body fat and low muscle mass (sarcopenic obesity). Pair BMI with waist circumference and grip strength for better assessment.