BMI Calculator
Calculate your Body Mass Index in metric or imperial units and see the category — along with an honest account of what BMI does not measure.
The same body gets a different verdict at a different height
Take a person and scale them uniformly — every dimension multiplied by the same factor, so the shape is unchanged. Mass grows with volume, which is the cube of height. BMI divides by the square. So the index rises in direct proportion to height for a body that has not changed shape at all:
| Height | Mass, same shape | BMI | Category |
|---|---|---|---|
| 1.50 m | 46.4 kg | 20.6 | healthy |
| 1.60 m | 56.3 kg | 22.0 | healthy |
| 1.70 m | 67.5 kg | 23.4 | healthy |
| 1.80 m | 80.2 kg | 24.7 | healthy |
| 1.90 m | 94.3 kg | 26.1 | overweight |
| 2.00 m | 110.0 kg | 27.5 | overweight |
Every row is the same person. The 1.9 m version is overweight and the 1.6 m version healthy, with identical proportions and identical body composition. Nothing about the body changed; only the arithmetic did.
The scale-free index is kg/m³, and nobody uses it
| Index | Variation across 1.5–2.1 m, fixed shape |
|---|---|
| kg / m2 | 1.400× |
| kg / m2.5 | 1.183× |
| kg / m3 | 1.000× — exactly flat |
Dividing by the cube is the only exponent that cancels the scaling entirely. It is not used because the resulting numbers are unfamiliar and every category would have to be redrawn.
None of which makes BMI useless. Quetelet devised it in the 1830s explicitly as a population statistic and said it should not be applied to individuals — and as a population statistic it works, because the scaling bias averages out when heights fall either side of the mean. It goes wrong precisely where it is most often used: on one person, especially a notably tall or short one.
| At this height | BMI 25 means |
|---|---|
| 1.55 m | 60.1 kg |
| 1.70 m | 72.2 kg |
| 1.85 m | 85.6 kg |
| 2.00 m | 100.0 kg |
How to use
- Enter your height and weight.
- Read your BMI and its category.
- Read the limitations before drawing conclusions.
- Discuss anything concerning with a clinician.
Frequently asked questions
How is BMI calculated?
Weight in kilograms divided by height in metres squared. In imperial units, weight in pounds divided by height in inches squared, multiplied by 703. The standard adult categories are under 18.5 underweight, 18.5 to 24.9 normal, 25 to 29.9 overweight, and 30 or above obese.
What does BMI actually measure?
Weight relative to height, and nothing else. It cannot distinguish muscle from fat, tell you where fat is carried, or account for bone density or frame size. A muscular athlete and a sedentary person of the same height and weight receive the same BMI, which is the clearest illustration of what the number leaves out.
Why is it still used if it is so crude?
Because it is cheap, quick and reproducible, and across large populations it correlates well enough with health outcomes to be useful for public health statistics. It was designed for populations rather than individuals, which is precisely where it gets misapplied.
Does BMI apply equally to everyone?
No. Health risks associated with a given BMI differ between populations — several health bodies use lower thresholds for people of South Asian, Chinese and other Asian ancestry, because cardiometabolic risk rises at a lower BMI. It is also not valid for children, who need age and sex percentiles, nor during pregnancy.
What is a better measure?
Waist circumference and the waist-to-height ratio both correlate better with health risk, because abdominal fat matters more than total weight. A common rule of thumb is keeping waist circumference under half your height. Body composition measurement is better still, and considerably less accessible.
Should I be worried about my BMI?
It is one data point, not a diagnosis. A number outside the normal range is worth mentioning to a clinician who can consider your body composition, blood pressure, blood markers, activity and history alongside it. Nothing here is medical advice.
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