BMI Calculator
Body Mass Index from height and weight, with the WHO band, the healthy weight range for your height, and where the index misleads.
BMI Calculator workspace
Parameters Configuration
Ready for calculations
Configure parameters on the left and press Calculate.
Using BMI Calculator
Pick metric or imperial on the unit toggle. Switching converts what is already in the fields rather than reusing the raw number.
Enter your weight, in kilograms or pounds depending on the toggle.
Enter your height, in centimetres or inches.
Read the index, the WHO category it falls in, and the weight range that would put you between 18.5 and 24.9 at that height.
Weight in kilograms divided by height in metres squared. Imperial entry divides pounds by inches squared and multiplies by 703, which is the unit conversion rather than a health constant. Categories are the WHO adult classification, and the healthy weight range shown is 18.5 to 24.9 mapped back onto the height you entered.
Body Mass Index compresses two measurements into a single number: your weight divided by the square of your height. That compression is the point of the index, and also its central weakness. It is cheap, reproducible and genuinely useful for comparing groups of people. It is far weaker as a statement about any one person.
The division, and what the 703 is for
In metric units the calculation is direct:
BMI = weight in kilograms / (height in metres x height in metres)Someone weighing 82 kg at 178 cm is 1.78 m tall, so 1.78 x 1.78 = 3.1684, and 82 / 3.1684 = 25.9. Under the World Health Organization's adult cut-offs that sits in the overweight band, which runs from 25.0 to 29.9 (underweight below 18.5, healthy 18.5-24.9, obese 30 and above).
Imperial entry takes pounds and inches and multiplies the result by 703. The 703 is not a health constant, it is the unit conversion that turns lb/in-squared into kg/m-squared. A person of 200 lb and 70 in comes out at 28.7 either way, because switching the toggle converts the values you already entered rather than reusing the raw digits — 70 read as centimetres instead of inches would give a very different answer.
The calculator also reports the weight span that would place you between 18.5 and 24.9 at your height. At 178 cm that span is 58.6 kg to 78.9 kg — nearly 20 kg wide, which is a useful corrective to the idea that there is one correct weight for a given height.
The index is wrong about two groups in opposite directions
BMI records how heavy you are, not what the weight is made of. A rugby forward carrying a great deal of muscle lands in the overweight or obese bands with single-digit body fat, and the index has no way to notice. Circumference-based estimates such as the body fat calculator at least respond to shape.
The error runs the other way in older adults. Muscle mass declines with age while fat is retained, so someone can lose a substantial amount of lean tissue, gain fat, and see their BMI barely move. A reassuring number in an eighty-year-old can conceal exactly the change that matters. This is why clinicians increasingly ask for waist circumference or grip strength alongside the index rather than instead of it.
A population statistic asked to do an individual's job
The bands come from mortality and disease data gathered across large cohorts. They describe where risk rises on average across thousands of people; they say nothing about whether your blood pressure, blood glucose or lipid profile is a problem. Two people at BMI 27 can have completely different metabolic health, and no arrangement of height and weight will separate them.
The thresholds are also not universal. A 2004 WHO expert consultation, published in *The Lancet*, concluded that many Asian populations show raised cardiometabolic risk at lower indices, and identified 23 and 27.5 as additional public health action points rather than replacing the standard cut-offs outright. If you have South or East Asian ancestry, read 23 as the point at which the conversation starts.
What this tool will not do
Adult cut-offs do not apply to anyone under 18; children are assessed against age-and-sex percentile charts because their proportions change as they grow. The index is also uninformative during pregnancy, when weight gain is expected and tells you nothing about composition. Everything is computed in your browser and nothing is transmitted; your entries may be kept in your own browser storage so a refresh does not wipe the form. Nobody with medical training has reviewed this page, and it is not a substitute for one.
Frequently asked
Does a BMI inside the healthy band mean I am healthy?
It means your weight-to-height ratio sits where population studies record the lowest average risk. It says nothing about your blood pressure, blood glucose, lipids, fitness or how much of your weight is muscle. Two people at the same index can be in very different metabolic health, and this calculation cannot tell them apart.
Which threshold applies if I have South or East Asian ancestry?
A 2004 WHO expert consultation published in The Lancet concluded that several Asian populations show raised cardiometabolic risk at lower indices, and identified 23 and 27.5 as additional public health action points alongside the standard 25 and 30. The tool notes the 23 figure so you can read it against your own result.
Why is the healthy weight range for my height so wide?
Because the healthy band spans 18.5 to 24.9, which at 178 cm works out as 58.6 kg to 78.9 kg — roughly 20 kg. That width is real rather than a rounding artefact, and it is a useful corrective to the idea that a given height has one correct weight.
Can I use this for a child or teenager?
No. Adult cut-offs do not transfer to anyone under 18, because children's proportions change as they grow and a fixed threshold would read normal development as a problem. Paediatric assessment uses BMI-for-age percentile charts specific to age and sex.