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BMI = Weight (kg) ÷ Height² (m²). For imperial: BMI = 703 × Weight (lbs) ÷ Height² (in²). Same formula used by WHO and all major health organisations.
BMI does not account for muscle mass, bone density, or fat distribution. Athletes may show high BMI despite low body fat. Use BMI as a screening guide, not a diagnosis.
Underweight: below 18.5 · Normal: 18.5–24.9 · Overweight: 25–29.9 · Obese Class I: 30–34.9 · Obese Class II: 35–39.9 · Obese Class III: 40+
Waist circumference, blood pressure, cholesterol, blood sugar, and physical activity are also key health indicators. Consult a doctor for a complete health assessment.
Body mass index is weight in kilograms divided by height in metres squared. Someone who is 1.70 m tall and weighs 68 kg has a BMI of 68 ÷ (1.70 × 1.70), which is about 23.5. In imperial units the same relationship is written as weight in pounds divided by height in inches squared, multiplied by 703.
The index was devised in the nineteenth century by the Belgian statistician Adolphe Quetelet, who was studying the characteristics of populations rather than assessing individuals. That origin explains both its strengths and its weaknesses. It is a fast, cheap, reproducible measure that needs nothing but a tape and a scale, which is why public health systems still rely on it. It was never designed to describe one person's health, and it does not.
The widely used international categories are these:
| Category | BMI range |
|---|---|
| Underweight | Below 18.5 |
| Healthy range | 18.5 to 24.9 |
| Overweight | 25.0 to 29.9 |
| Obesity | 30.0 and above |
These thresholds were derived largely from European-ancestry populations, and research over the past two decades has shown they do not describe cardiometabolic risk equally well everywhere. People of South Asian, and to varying degrees other Asian, descent tend to carry a higher proportion of body fat and more visceral fat around the organs at the same BMI, and show elevated risk of type 2 diabetes and cardiovascular disease at lower thresholds.
For this reason, guidance used across India and much of Asia applies lower cut-offs, commonly treating 23.0 as the point at which risk begins to rise and 25.0 as the threshold for obesity, rather than 25 and 30. If your background is South Asian, the standard international table may place you in a category that understates the picture, and the Asian thresholds are the more relevant reference to discuss with a clinician.
BMI knows two numbers about you and nothing else. It cannot distinguish between muscle and fat, so trained athletes and physically heavy-working people are routinely classified as overweight while being in excellent condition. It cannot see where weight is carried, and abdominal fat is more strongly associated with metabolic risk than the same mass distributed elsewhere. It says nothing about blood pressure, blood sugar, lipid profile, fitness, sleep or diet quality, all of which are better predictors of health than the ratio itself.
It is also inappropriate or unreliable for several groups. It should not be applied to children and teenagers, who are assessed using age-and-sex percentile charts because normal body composition changes as they grow. It does not apply during pregnancy. It is unreliable for older adults, who lose muscle mass and height with age, and for people with limb differences, significant fluid retention, or certain medical conditions. In short, a single number cannot carry the weight that is often placed on it.
If you want a fuller picture without clinical equipment, two simple measurements add a lot.
Waist circumference is measured at the midpoint between the lowest rib and the top of the hip bone, at the end of a normal breath out. It captures abdominal fat, which BMI cannot. General thresholds used in many guidelines sit around 94 cm for men and 80 cm for women, with lower values, often around 90 cm and 80 cm, applied for South Asian populations.
Waist-to-height ratio is even simpler: waist divided by height, in the same units. A ratio below roughly 0.5 is the commonly cited guide, which conveniently means keeping your waist under half your height. It requires no reference table, works across ages and populations more consistently than BMI, and takes ten seconds.
Two practical suggestions. First, treat BMI as a trend rather than a snapshot. A single reading moves with hydration, time of day, recent meals and the accuracy of your height measurement, so a change of a few tenths means very little. What a trend over months tells you is more useful than what today's number says.
Second, be wary of using it as a target. The index was built to describe populations, and optimising a population statistic for an individual can lead somewhere unhelpful. Measures of what your body can do, and clinical markers of how it is functioning, are more meaningful goals than a ratio. If thinking about weight or body image causes you distress, or if you find yourself preoccupied with these numbers, that is worth raising with a healthcare professional too. It is a common experience and a reasonable thing to seek support for.
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BMI is weight in kilograms divided by height in metres squared. A person 1.70 m tall weighing 68 kg has a BMI of about 23.5. In imperial units it is weight in pounds divided by height in inches squared, multiplied by 703.
Yes. The standard international cut-offs of 25 for overweight and 30 for obesity were derived largely from European-ancestry populations. Guidance used across India and much of Asia commonly uses 23 as the point where risk begins to rise and 25 as the obesity threshold, because risk of type 2 diabetes and cardiovascular disease tends to appear at a lower BMI.
No. BMI cannot distinguish muscle from fat, so people with a lot of muscle are frequently classified as overweight while being in very good condition. Waist circumference or a body composition assessment gives a more useful picture in those cases.
No, not with the adult categories. Children and teenagers are assessed against age-and-sex specific percentile charts because normal body composition changes throughout growth. A paediatrician or the relevant growth chart should be used instead.
Waist circumference and waist-to-height ratio are both simple and add information BMI cannot provide, because they capture abdominal fat. Waist-to-height ratio is the easiest: keep your waist measurement under half your height, which needs no reference table.
Not on its own. BMI is a population screening tool, not a diagnosis, and it says nothing about blood pressure, blood sugar, lipids, fitness or diet. If your result concerns you, discuss it with a doctor who can interpret it alongside your other markers and your history.
No. The calculation runs entirely in your browser on your own device. Nothing you enter is transmitted, saved between visits or logged, and the page continues to work with your network disconnected.
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