BMI Body Mass Index Calculator

Body mass index (BMI) is a simple screening measure that relates body weight to height. It is used by clinicians, researchers and public health agencies worldwide as a quick way to classify a person as underweight, a healthy weight, overweight or living with obesity, and to track weight trends across populations. BMI does not measure body fat directly, so it is best understood as a starting point for a conversation about weight and health rather than a diagnosis.

The formula

BMI = weight (kg) / height (m) squared

• weight = body weight measured in kilograms
• height = standing height measured in metres, without shoes

Devised by the Belgian statistician Adolphe Quetelet in the 1830s, originally called the Quetelet Index; adopted by the World Health Organization in the 1990s as the standard international measure for classifying weight status in adults.

How the calculation works

The calculation divides body weight in kilograms by the square of height in metres. Squaring the height, rather than using it directly, corrects for the fact that taller people naturally weigh more even at a healthy build; without the square term, BMI would rise with height for no reason related to body fat. In countries that use pounds and inches, the equivalent formula multiplies weight in pounds by 703, then divides by height in inches squared.

The result is a single number, typically between about 15 and 45 for adults, expressed in kilograms per square metre (kg/m2). It says nothing about where fat is stored on the body, how much of the weight is muscle versus fat, or a person's overall fitness. It is a population-level screening tool, refined over decades of epidemiological research linking BMI bands to average health outcomes across large groups.

How to read your result

The World Health Organization and the US Centers for Disease Control and Prevention use the same standard adult classification, applicable to non-pregnant adults aged 20 and over of most ethnic backgrounds. Each band reflects the average risk of weight-related health conditions, such as type 2 diabetes, high blood pressure and cardiovascular disease, observed at a population level; individual risk varies.

• A BMI in the healthy weight range is generally associated with the lowest average risk of weight-related disease in large population studies.
• Health risk begins rising gradually even within the upper part of the healthy weight band for some conditions, and continues rising through the overweight and obesity bands.

Using BMI as a starting point, not an endpoint

Clinicians rarely rely on a single BMI reading in isolation. In practice it functions as an initial screening step: a result outside the healthy range prompts a fuller assessment that might include waist circumference, blood pressure, fasting glucose and family history before any treatment decision is made. A pooled analysis of nearly 900,000 adults published in The Lancet in 2009 found that both being markedly underweight and having a BMI well above 30 were associated with higher all-cause mortality, with the lowest risk clustering around 22.5 to 25 kg/m2, broadly supporting the standard healthy weight band even though the relationship is not identical in every subgroup.

The number that matters for an individual also depends on trend, not just a single snapshot. A BMI that has climbed by several points over a few years signals a different situation than the same BMI that has stayed stable for a decade, even though both readings might land in the same category on a given day. Repeating the calculation every few months and watching the direction of travel is generally more informative than fixating on one result.

• If your BMI falls outside the healthy range, treat it as a prompt to look at the trend over time and discuss it with a clinician, not as a standalone diagnosis.
• Waist circumference or waist-to-height ratio can help clarify a result when BMI and visible body composition seem to disagree, since they capture abdominal fat that BMI cannot see.

Why cut-offs differ for Asian populations

Large studies across Asia found that people of Chinese, Indian, Japanese and other Asian ancestries tend to develop type 2 diabetes, hypertension and cardiovascular disease at lower BMI values than the standard cut-offs suggest, partly because they carry more body fat, and more visceral abdominal fat, at the same BMI as European populations. In response, a WHO expert consultation proposed alternative action points for Asian populations.

Several national health authorities, including in Singapore, Japan, India and China, have adopted these or similar lower thresholds for clinical screening and public health guidance. The standard WHO/CDC cut-offs remain the global default in most calculators and are used above; anyone of Asian descent may want to interpret a result in the 23 to 27.5 range with this context in mind, and discuss it with a clinician familiar with the population-specific guidance used locally.

Limitations

• BMI cannot distinguish fat mass from muscle mass. Bodybuilders, strength athletes and very muscular individuals are frequently classified as overweight or obese despite low body fat.
• BMI is not used to classify children and teenagers. Growth is assessed instead with age- and sex-specific BMI-for-age percentile charts, because body composition changes rapidly during growth.
• BMI has not been validated for use during pregnancy or breastfeeding, when weight gain is expected and healthy.
• BMI does not indicate where fat is distributed. Abdominal fat carries more cardiometabolic risk than fat stored elsewhere, and two people with the same BMI can have very different waist circumferences and health risk.
• The standard cut-offs were derived mainly from European-ancestry populations. Asian, and to a lesser extent other, populations may face elevated health risks at lower BMI values than the standard thresholds indicate, as described above.
• BMI may be less reliable in older adults, who often lose muscle mass while retaining or gaining fat, and in people with limb loss or amputation, where standard height-weight assumptions no longer hold.

Frequently asked questions

What counts as a healthy BMI?

For most adults, a BMI between 18.5 and 24.9 kg/m2 falls in the healthy weight range used by the WHO and CDC. This range is associated with the lowest average risk of weight-related disease across large populations, though individual health depends on many other factors.

Why do some countries use lower BMI cut-offs for people of Asian descent?

Population studies found that people of Asian ancestry tend to have more body fat, and more abdominal fat, at a given BMI than people of European ancestry, and that diabetes and cardiovascular risk rise at lower BMI values. A WHO expert consultation proposed lower action points, 23 for overweight and 25 for obesity, to reflect this, and several Asian health authorities have adopted them.

Is BMI accurate for athletes and very muscular people?

Not reliably. Because BMI cannot tell muscle from fat, athletes with high muscle mass are often classified as overweight or obese even when their body fat percentage is low. Measures such as waist circumference or body composition scanning give a more accurate picture for this group.

Should I use BMI to judge my own health?

BMI is a screening tool, not a diagnosis. It is useful for tracking trends over time and flagging when a fuller assessment may help, but a single BMI number should be interpreted alongside waist circumference, blood pressure, blood glucose, family history and a clinician's assessment.

Does a single BMI reading tell my doctor everything they need to know?

No. Clinicians typically treat BMI as a first screening step, followed by measurements such as waist circumference, blood pressure and blood tests, and by looking at how the number has changed over time, before drawing conclusions about individual health.

References

1. Centers for Disease Control and Prevention. About Body Mass Index (BMI).
2. Centers for Disease Control and Prevention. BMI Frequently Asked Questions.
3. World Health Organization. Obesity and overweight fact sheet.
4. WHO Regional Office for the Western Pacific, IASO, IOTF. The Asia-Pacific Perspective: Redefining Obesity and its Treatment, 2000.