How to calculate your BMI
- Choose metric or imperial units.
- Enter your height and weight in the fields for that system.
- Read your BMI figure and the category it falls into.
- Compare it against the healthy weight range shown for your height.
- Treat the result as a rough screening number, not a diagnosis.
What this calculator shows
- BMI to one decimal place, calculated as weight in kilograms divided by height in metres squared.
- The standard World Health Organization category for that figure.
- The weight range that corresponds to a BMI of 18.5 to 24.9 at your height.
- All four category boundaries so you can see where you sit relative to them.
- Metric and imperial input, including stones and pounds.
- Calculation in your browser. No height or weight is ever transmitted.
What BMI actually measures
Body mass index is your weight in kilograms divided by the square of your height in metres. That is the whole formula. It was devised in the 1830s by a Belgian statistician studying populations, and it was never intended as a measure of an individual's health.
What it does well is give a quick, cheap, comparable number across large groups. What it cannot do is tell you anything about what your weight is made of, where it sits on your body, or how fit you are, and those are the things that actually relate to health outcomes.
The standard categories
- Below 18.5: underweight
- 18.5 to 24.9: healthy weight
- 25 to 29.9: overweight
- 30 and above: obese, subdivided into three classes
These boundaries are round numbers chosen for convenience, not thresholds where something changes. A BMI of 24.9 and one of 25.1 describe practically identical bodies, and treating one as fine and the other as a problem is not meaningful.
Where BMI is known to mislead
Muscle is denser than fat, so muscular people are routinely classified as overweight or obese while carrying very little fat. A great many professional athletes have a BMI above 25. In the other direction, someone with low muscle mass can sit in the healthy band while carrying a high proportion of body fat, sometimes called normal-weight obesity.
It also performs differently across populations. Health risks associated with a given BMI appear at lower figures in people of South Asian descent, which is why several countries use a lower overweight threshold for those groups. Older adults lose height and muscle, which shifts BMI without any change in health. And it says nothing at all about where fat is distributed, though abdominal fat is the pattern most strongly linked to metabolic risk.
Measures that add more
Waist circumference is a better single indicator of abdominal fat than BMI, and it needs nothing but a tape measure. Waist-to-height ratio, keeping your waist under half your height, is simple and correlates better with risk in most studies. Blood pressure, resting heart rate, blood glucose and lipid panels tell you far more than any body measurement, and only a clinician can interpret them properly.
A note on how to use this
BMI is useful as one rough datapoint tracked over time, and unhelpful as a verdict. If the number concerns you, that is a reason to talk to a doctor rather than to change your eating on the strength of a calculation. Rapid weight change, restrictive dieting and fixating on a single figure all carry their own risks.
Everything here is worked out in your browser and nothing is transmitted, which matters for a measurement most people consider private. The percentage calculator and unit converter handle the arithmetic if you need to convert measurements for a form.