BMI Calculator

The two BMI formulas

Body mass index is weight divided by height squared. It was devised in the 1830s by the Belgian statistician Adolphe Quetelet as a way of describing populations, and it is still expressed in kilograms per square metre. The imperial version reaches the same number by a different route, using a conversion constant.

In both formulas, weight is total body mass and height is standing height. The metric version needs height in metres, so this calculator asks for centimetres and divides by 100 before squaring. The imperial version asks for feet and inches separately and converts them to total inches (feet × 12 + inches), so both fields need a value before a result appears.

Where 703 comes from

The constant is not arbitrary. One pound is 0.45359237 kg and one inch is 0.0254 m, so dividing 0.45359237 by 0.0254² gives 703.07. Rounding it to 703 is the standard convention and shifts the result by well under a tenth of a point, which is why metric and imperial entries for the same person agree.

Worked example in metric

Worked example in imperial

Adult BMI categories

These are the World Health Organization cut-offs, also used by the National Institutes of Health, and they are the exact ranges this calculator applies. They are intended for adults aged 20 and over.

Two details are worth knowing. First, the result is rounded to one decimal place and the category is then read off that rounded number, so the figure on screen and the label beside it always agree with the table above. Second, the WHO subdivides obesity into class I (30 to 34.9), class II (35 to 39.9) and class III (40 and above); this calculator reports a single Obesity label rather than the class.

What BMI is actually good for

BMI earns its place because it is cheap, repeatable and needs nothing but a tape measure and a scale. Across large groups it tracks reasonably well with body fat percentage and with the risk of type 2 diabetes, cardiovascular disease, several cancers and all-cause mortality. That makes it a workable instrument for public health surveillance, for comparing populations over time, and for research where measuring body composition directly on tens of thousands of people would be impossible.

For one person it is best understood as a first-pass screen: a number that suggests whether a longer conversation about weight is worth having, not an answer in itself. Tracking your own BMI over years, where height is stable, is more informative than any single reading, because the change reflects a change in weight rather than a comparison against a population average.

Where BMI falls short

BMI has no way of knowing what the weight is made of. Muscle is denser than fat, so a well-trained rugby player and a sedentary person of the same height and weight receive an identical BMI despite very different body composition. Athletes with high muscle mass are routinely classified as overweight or obese by BMI alone.

It also ignores where fat sits. Visceral fat around the abdomen carries substantially more metabolic risk than fat on the hips and thighs, and BMI cannot distinguish the two. This is why clinicians often pair it with waist circumference or waist to height ratio, which pick up central adiposity that BMI misses entirely.

Because the formula divides by height squared while human bodies do not scale that neatly, BMI also tends to read slightly high in taller people and slightly low in shorter people of otherwise similar build.

Groups where the standard cut-offs do not fit

• Children and teenagers. Body composition changes rapidly with growth, so under-20s are assessed against age and sex specific percentile charts, not the adult table above. Do not apply these categories to a child.
• Older adults. Muscle mass declines and height is lost with age, both of which push BMI in misleading directions. Some evidence suggests a slightly higher BMI is protective in later life.
• Pregnancy. BMI is not a meaningful measure during pregnancy. Pre-pregnancy BMI is what maternity guidance uses.
• Athletes and very muscular people. High lean mass inflates BMI without the associated risk.
• Different ethnic groups. Cardiometabolic risk appears at lower BMI values in South Asian, Chinese and other East and South East Asian populations. A WHO expert consultation identified additional public health action points at 23 and 27.5, and UK NICE guidance recommends treating 23.0 as the overweight threshold and 27.5 as the obesity threshold for South Asian, Chinese, other Asian, Middle Eastern, Black African and African Caribbean groups. This calculator applies the standard thresholds, so read your result with that adjustment in mind if it applies to you.

Frequently asked questions

Is a BMI of 26 unhealthy?

On its own it tells you very little. A BMI of 26 sits in the overweight band, but the same number can describe a muscular athlete and someone carrying significant abdominal fat. Blood pressure, blood lipids, blood glucose, waist circumference and activity level are all better indicators of actual health than the index by itself.

Do men and women use different BMI ranges?

No. The adult cut-offs are identical for both, which is one of the known weaknesses of the measure, since women typically carry a higher body fat percentage than men at the same BMI.

Why do I get slightly different numbers on different sites?

Usually rounding. Some tools round the input height first, some round the result to whole numbers, and imperial calculators may use 703 or the more precise 703.07. Differences of a tenth of a point are normal and carry no meaning.

What should I measure to get an accurate reading?

Weigh yourself in the morning, after using the bathroom and before eating, wearing little or no clothing, and measure height standing straight without shoes. Day to day weight swings of one or two kilograms from food and fluid are ordinary and will move BMI by a few tenths.

What is a better measure than BMI?

No single number replaces it, but waist to height ratio (keeping waist below half your height) captures central fat that BMI misses and needs only a tape measure. Body composition methods such as DEXA scanning are more accurate again, at the cost of equipment and money. Most clinicians use several measures together rather than any one.

Not medical advice

Everything on this page is general information about how body mass index is calculated and interpreted. It is not medical advice, it is not a diagnosis, and no calculator can account for your history, medication, body composition or the results of any test. If your result concerns you, or if you are planning to change your weight, a doctor, registered dietitian or other qualified clinician is the right person to interpret the number in the context of your own health.