BMI Calculator

Body Mass Index squeezes two measurements into one number: your weight divided by the square of your height. That squeezing is the whole idea of the index, and it's also its main weakness. BMI is cheap, reproducible and genuinely useful for comparing groups of people. As a statement about any one person, it's far weaker.

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. That number is just the unit conversion that turns lb/in-squared into kg/m-squared; it has nothing to do with health. A person of 200 lb and 70 in comes out at 28.7 either way. When you flip the toggle, the calculator converts the values you've already entered instead of reusing the raw digits, which matters: 70 read as centimetres instead of inches would give a very different answer.

The calculator also shows the weight range that would put you between 18.5 and 24.9 at your height. At 178 cm that's 58.6 kg to 78.9 kg, nearly 20 kg wide. It's a useful corrective if you'd assumed there was one correct weight for a given height.

The index is wrong about two groups in opposite directions

BMI records how heavy you are, but not what that weight is made of. A rugby forward carrying a lot 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.

In older adults the error runs the other way. 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 hide exactly the change that matters. That's why clinicians increasingly ask for waist circumference or grip strength to go with the index.

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 tell you nothing about whether your own 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 aren't universal either. A 2004 WHO expert consultation, published in *The Lancet*, concluded that many Asian populations show raised cardiometabolic risk at lower indices. It identified 23 and 27.5 as additional public health action points, without replacing the standard cut-offs outright. If you have South or East Asian ancestry, read 23 as the point where the conversation starts.

What this tool won't do

Adult cut-offs don't 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 calculated in your browser and nothing is sent anywhere, though your entries may be kept in your own browser storage so a refresh doesn't wipe the form. Nobody with medical training has reviewed this page, and it can't stand in for someone who has.