BMI Calculator

How this is calculated

BMI is weight in kilograms divided by height in metres squared. Everything you enter is converted to metric first - pounds are multiplied by 0.45359237 and inches by 2.54, both exact definitions - so the answer is identical whichever units you choose. The healthy range shown is the weight you would be at a BMI of 18.5 and at a BMI of 25, converted back into your own units. Those two thresholds are the World Health Organization's adult classification, along with 30 for obesity. They are a population-level convention rather than a measurement of any individual, and the boundaries are round numbers chosen for convenience - there is no physical difference between a BMI of 24.9 and 25.1. The range is far more useful than the index. 'Your BMI is 24.3' is hard to act on; 'the healthy range for your height is 121.7 to 164.4 pounds' is the same fact in units you already think in. What BMI does NOT measure is body composition. It cannot distinguish muscle from fat, and it does not know where the fat is - which matters, because abdominal fat carries more risk than the same weight elsewhere. A muscular person is routinely classified overweight by it and a sedentary person with little muscle can sit in the middle of the range. Waist measurement adds most of what BMI is missing, and costs one tape measure. It was never designed for individual diagnosis. Adolphe Quetelet devised the ratio in the 1830s to describe POPULATIONS, and it was not intended as a measure of individual fatness or health. It is still used because it is cheap and reproducible, not because it is precise. This calculator is for adults. It does not apply to children or teenagers, who are assessed against age-and-sex percentile charts instead, and it is not meaningful during pregnancy.

Common questions

What do the numbers mean?

The World Health Organization's adult bands are: under 18.5 underweight, 18.5 to 25 healthy weight, 25 to 30 overweight, and 30 and above obese. They are a population-level convention, not a diagnosis, and the boundaries are round numbers picked for convenience - nothing physical changes between 24.9 and 25.1. Treat a number near a boundary as being near a boundary, not as belonging to one side of it.

I lift weights and it says I'm overweight. Is it wrong?

For you, probably yes. BMI cannot distinguish muscle from fat - it only knows your total weight and your height. Muscle is denser than fat, so a well-muscled person carrying very little fat is routinely classified overweight or even obese by this measure. This is the single best-known limitation of BMI and it is a real one, not a technicality.

What should I use instead, or as well?

Waist measurement adds most of what BMI is missing, because WHERE fat sits matters as much as how much there is - abdominal fat carries more metabolic risk than the same weight on hips or legs. Waist-to-height ratio is a common companion measure and needs nothing but a tape measure. Neither replaces a conversation with a doctor who can see the whole picture.

Does BMI work the same for everyone?

No. The standard thresholds were derived largely from European-descent populations, and several health bodies use lower cut-offs for people of South Asian, Chinese and other ancestries, where metabolic risk appears at a lower BMI. Age and sex also change what a given number implies. This calculator shows the standard WHO bands; whether they are the right bands for you is a question for a clinician.

Can I use this for my child?

No. Children and teenagers are assessed against age-and-sex percentile charts, because a growing body's healthy ratio changes year by year. A raw BMI figure for a child is not interpretable against the adult bands, and this calculator should not be used that way. It is also not meaningful during pregnancy.

Why does it show the weight range rather than just a category?

Because the range is the part you can do something with. 'Your BMI is 24.3' is an abstraction; 'the healthy range for your height is 121.7 to 164.4 pounds' is the same fact expressed in the units on your bathroom scale. It also makes the width of the range visible - it is usually wider than people expect, which is worth seeing.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the BMI Calculator at https://www.bfcbrilliance.com/tools/bmi-calculator.

What I entered:
- Weight: ___
- Weight in: ___
- Height: ___
- Height in: ___

What it calculated:
- Your BMI: ___
- Healthy range for your height — from: ___
- — up to: ___
- Distance to the top of that range: ___
- Your height in metres: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Keep this general and do not give medical advice — where it matters, tell me what to raise with a clinician.

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