What Is BMI? Meaning, Formula and Asian Cut-offs

This article is general information, not medical advice. Consult a qualified doctor for an assessment of your own health and weight.

BMI, or body mass index, is a simple number that estimates whether a person’s weight is healthy for their height. It is calculated by dividing weight in kilograms by height in metres squared, and is widely used by health services as a quick screening tool to classify adults as underweight, healthy weight, overweight or obese. It offers a fast, low-cost snapshot of a population’s weight status.

Because it needs only height and weight, BMI is easy to measure and compare across large groups, which is why the World Health Organization and national health bodies rely on it. However, it is a screening indicator rather than a complete diagnosis, and it carries important limitations – especially for Indian and other Asian populations, where the standard thresholds understate risk.

How is BMI calculated?

The formula is straightforward: BMI equals weight in kilograms divided by height in metres, squared. For example, a person who weighs 60 kg and stands 1.65 m tall has a BMI of 60 divided by (1.65 x 1.65), which works out to roughly 22. Most online calculators and health apps perform this calculation automatically once you enter your height and weight.

The resulting number is then compared against standard categories to indicate whether a person’s weight may pose a health risk. Because the same formula is used everywhere, BMI allows straightforward comparison between individuals and across whole populations.

What are the standard BMI categories?

Under the World Health Organization’s general classification for adults, BMI is grouped into bands. The table below sets out the standard cut-offs alongside the lower thresholds recommended for Asian populations.

Why are BMI cut-offs lower for Asians and Indians?

A WHO Expert Consultation reviewed evidence from across Asia and found that many Asian populations develop type 2 diabetes and cardiovascular risk factors at a lower BMI than the standard overweight threshold of 25. It concluded that, in some Asian groups, a given BMI can reflect a higher proportion of body fat than in European populations, and proposed additional public-health action points at BMI levels such as 23 and 27.5.

India’s ICMR-National Institute of Nutrition applies this thinking in its Dietary Guidelines for Indians, treating a BMI of 18.5 to 22.9 as healthy, 23 to 27.5 as overweight and above 27.5 as obese. For Indians, therefore, a BMI regarded as “normal” by the global standard may already signal elevated risk, which is why the lower cut-offs matter.

What are the limitations of BMI?

BMI is useful, but it is a blunt instrument. It does not distinguish between fat and muscle, so a very muscular athlete can register as “overweight” despite low body fat. Nor does it show where fat is stored, even though abdominal fat carries particular health risks. It can also be less reliable for older adults, pregnant women, children and certain ethnic groups.

For these reasons, health professionals often combine BMI with other measures. Waist circumference and the waist-to-hip ratio help capture abdominal fat, and are especially relevant for Indians, who are prone to central obesity even at a modest BMI.

Why does BMI matter for health?

Despite its limitations, BMI remains a valuable early-warning tool. The World Health Organization identifies overweight and obesity as major risk factors for non-communicable diseases including heart disease, stroke, type 2 diabetes and some cancers. Tracking BMI at a population level helps governments understand the scale of these risks and plan public-health responses.

For an individual, BMI is best seen as a starting point for a conversation with a doctor rather than a final verdict. A reading outside the healthy range is a prompt to look more closely at diet, physical activity and other risk factors, not a diagnosis in itself.

Does BMI work the same way for children and older adults?

Not exactly. For children and adolescents, BMI is interpreted differently from adults: it is compared against age- and sex-specific growth references rather than fixed cut-offs, because a child’s body composition changes as they grow. A number that is healthy at one age may not be at another.

For older adults, BMI can also be less reliable, as muscle mass naturally declines and height can decrease with age. In both groups, a simple BMI reading should be interpreted with extra care and ideally alongside professional assessment. This underlines the broader point that BMI is a screening tool, not a one-size-fits-all verdict on health.

How does BMI differ from body fat percentage?

BMI and body fat percentage measure different things. BMI is calculated purely from height and weight, so it cannot tell how much of a person’s weight is fat and how much is muscle, bone or water. Body fat percentage, by contrast, estimates the share of total body weight that is fat, and is measured using methods such as skinfold callipers or specialised scales.

This is why two people with the same BMI can have very different amounts of body fat. A muscular person may have a high BMI with low fat, while someone with little muscle may have a normal BMI yet a high proportion of fat – a pattern sometimes described as being “thin on the outside, fat on the inside”, which is of particular concern in Indian populations. Body fat measures add useful detail, but BMI remains the quick, accessible first check.

Why is BMI especially important in India?

India faces a growing burden of non-communicable diseases such as type 2 diabetes and heart disease, often appearing at lower body weights than in many Western countries. Because Indians tend to carry more body fat, and more abdominal fat, at a given BMI, the standard global thresholds can give false reassurance. This is precisely why the ICMR-National Institute of Nutrition adopts the lower Asian cut-offs in its Dietary Guidelines for Indians.

Using the lower thresholds means more Indians are flagged as overweight or at risk at an earlier stage, creating an opportunity to act through diet and activity before disease develops. Public-health experts see this earlier warning as an advantage, given how closely excess weight is linked to India’s diabetes and cardiovascular challenges.

A worked example: how to read your BMI

Suppose an adult of Indian origin weighs 72 kg and is 1.70 m tall. Their BMI is 72 divided by (1.70 x 1.70), which is about 24.9. Under the standard WHO bands this sits within the healthy range. But under the Asian cut-offs used by the ICMR-NIN, a BMI of 24.9 falls into the overweight category, signalling that some lifestyle attention may be warranted.

This example shows why the choice of cut-off matters so much for Indians, and why the same number can carry a different meaning depending on the reference used. It also underlines that BMI is a prompt for reflection, not a diagnosis on its own.

How can you use BMI sensibly?

Treat BMI as one indicator among several. Calculate it accurately, compare it against the appropriate cut-offs – the lower Asian thresholds if you are of Indian or Asian origin – and consider it alongside your waist measurement, activity level and overall health. The ICMR-NIN guidance encourages regular physical activity, balanced meals, and steps to prevent abdominal obesity. If your BMI falls outside the healthy range, or you are unsure how to interpret it, seek advice from a qualified doctor. This article is general information and not a substitute for professional medical guidance.