BMI Calculator for Adults: WHO Ranges, Formula, and Limits

Adult BMI is weight in kilograms divided by height in meters squared. An adult who is 170 cm tall and weighs 65 kg has a BMI of 22.5, which the World Health Organization places in the normal range. This post gives the formula and a calculator, works through the arithmetic step by step, sets the WHO table next to the lower cutoffs used across much of Asia, and spends the second half on the thing most BMI pages skip: the specific cases where the number is wrong about you.

Where the BMI formula comes from

BMI (body mass index) compares weight against height. The formula is metric, so height goes in meters even though most people know their height in centimeters:

height_m = height_cm / 100
BMI = weight_kg / (height_m x height_m)

One division to convert the height, one multiplication to square it, one more division for the result. BMI carries no unit and is normally written to one decimal place.

The square in the denominator surprises people. Volume scales with the cube of a linear dimension, so a naive model would divide weight by height cubed. Human bodies do not scale that way. Adults who are taller are not simply scaled-up versions of shorter adults, and when the Belgian statistician Adolphe Quetelet studied population measurements in the 1830s he found that adult weight tracked height squared far more closely than height cubed. Dividing by the square therefore produces a number that is roughly independent of height, which is the whole point: it lets you compare a 155 cm adult with a 190 cm adult on one scale. The ratio was known as the Quetelet index for well over a century.

The modern name arrived in 1972, when Ancel Keys and colleagues tested several relative weight indices against measured body fat and concluded that weight over height squared was the best of the simple ratios. They renamed it the body mass index. Keys was explicit about the scope of the tool: it was proposed for population studies, not for judging an individual. That original caveat has been quietly dropped in the decades since, and recovering it is most of what the rest of this post is about.

Set your own height and weight below. The result updates as you type, and the category label follows the WHO adult standard.

Two worked examples with the same three steps

Start with an adult who is 170 cm tall and weighs 65 kg.

1. Convert the height to meters: 170 / 100 = 1.70
2. Square it: 1.70 x 1.70 = 2.89
3. Divide the weight by that: 65 / 2.89 = 22.49
4. Round to one decimal place: 22.5

A BMI of 22.5 sits inside the WHO normal range of 18.5 to 24.9, and it also stays under the stricter Asian overweight cutoff of 23.0. On both tables, this result is unremarkable.

The second example shows why an unremarkable result is not the same as a complete answer. Take two people who are both 175 cm and 80 kg. The steps are identical: 1.75 x 1.75 = 3.0625, then 80 / 3.0625 = 26.1. Both are labelled overweight by the WHO table and obese by the Asia-Pacific table. Their bodies have almost nothing else in common.

• Person A is 28, trains with weights four times a week, and has a body fat percentage around 15%. Most of the 80 kg is skeletal muscle. Waist circumference 82 cm.
• Person B is 62, has not done resistance training in twenty years, and has a body fat percentage around 32%. Muscle mass has declined with age and the fat has settled around the abdomen. Waist circumference 104 cm.

BMI returns 26.1 for both, because BMI cannot see the difference. It has two inputs and neither of them is body composition. Person A's result is an artifact of muscle; Person B's result is understating an abdominal fat problem that a tape measure would catch in ten seconds. Same number, opposite meaning.

WHO BMI categories for adults

The classification below is the WHO standard for adults, and it is what the calculator on this page reports.

WHO splits the obese band into three classes, because risk keeps climbing well past the 30.0 boundary:

Two details are easy to miss. The bands describe adults, and WHO defines them for the general adult population rather than as a diagnosis for any one person. And the boundaries are hard edges applied to a soft measurement: 24.9 and 25.0 are neighbours, while the underlying risk curve is continuous. A result that lands within a rounding step of a boundary should be read as sitting on the line, not as a verdict that flipped. Reference: the WHO fact sheet on obesity and overweight.

Why Asian BMI cutoffs are lower (23 and 25)

The same BMI does not carry the same risk in every population. At a given BMI, people of Asian descent tend to carry a higher percentage of body fat, and more of it around the abdomen, than people of European descent. The rise in risk for type 2 diabetes and cardiovascular disease starts at a lower number. The WHO/IASO/IOTF Asia-Pacific guidance responded by shifting the thresholds down: overweight begins at 23.0 and obesity at 25.0, five full points below the international obesity cutoff.

Placing the two tables side by side shows where they disagree. The gap is entirely in the middle of the range, which is exactly where most adults land.

Two quick cases make the disagreement concrete. An adult who is 165 cm and 63 kg has a BMI of 23.1, which is normal internationally and overweight in the Asia-Pacific table. An adult who is 180 cm and 85 kg has a BMI of 26.2, which is overweight internationally and obese in the Asia-Pacific table. Neither person changed. Only the reference population did.

A 2004 WHO expert consultation reviewed this evidence and reached a compromise that still causes confusion: it retained the international classification so that countries could keep comparing data, while recommending additional public health action points at BMI 23.0 and 27.5 for Asian populations. National bodies then went their own way. Korea and Japan both treat 25.0 and above as obesity in national guidance, so a clinic in Seoul or Tokyo and a WHO summary table can describe an identical person in different words without either being wrong. Before you read anything into a BMI result, find out which standard produced the label.

What BMI cannot tell you, and what to measure alongside it

BMI takes two numbers and returns a third. Everything it fails at follows from what those two numbers leave out.

• Muscle versus fat. BMI counts every kilogram identically. Dense skeletal muscle pushes the result up exactly as excess fat does, so trained athletes and manual workers with high muscle mass are routinely labelled overweight or obese with no excess fat at all. BMI overestimates body fat in muscular people.
• Muscle loss. The same blindness runs the other way. Adults who have lost muscle mass, most often with age, can hold a perfectly normal BMI while carrying a high proportion of body fat. This pattern has a name, sometimes called normal weight obesity, and BMI is structurally incapable of detecting it.
• Where the fat sits. Fat stored around the abdomen and internal organs carries more metabolic risk than fat on the hips and thighs. BMI treats both the same because it never learns where anything is.
• Sex. At an identical BMI, women on average carry a higher body fat percentage than men. One table serves both sexes, so the same cutoff means slightly different things depending on who is standing on the scale.
• Age. Body composition shifts across a lifetime, and muscle mass tends to decline after roughly 60. There is evidence that the lowest-risk BMI band sits a little higher in older adults than in younger ones, which makes a strict reading of the adult table a poor fit at both ends of the age range.
• Pregnancy. BMI is not valid during pregnancy. Weight gain is expected and monitored against pregnancy-specific guidance instead.
• Children and adolescents. The adult cutoffs of 18.5, 25, and 30 do not apply. A growing body's composition changes constantly, so a child's BMI is compared against age and sex specific growth references and reported as a percentile or z-score.
• Ethnicity. As the previous section covered, one global set of boundaries cannot fit every population.

Three cheap measurements fill most of these gaps. Waist circumference captures abdominal fat directly, and it is the fastest way to separate Person A from Person B in the earlier example. Waist-to-height ratio is even simpler and needs no reference table: a common guideline is to keep your waist under half your height, which means a ratio below 0.5 at any height. Body fat percentage is the most direct measure of the thing BMI is trying to approximate, though the accurate methods such as DEXA require a clinic, and consumer bioimpedance scales vary enough that their trend over weeks is more informative than any single reading.

Used together, these give a picture BMI alone cannot. A BMI of 26 with a waist-to-height ratio of 0.45 tells a very different story from a BMI of 24 with a ratio of 0.58.

This page is not medical advice, and BMI is not a diagnosis. It is a screening measure designed for populations, borrowed for individual use. A BMI value is a reason to look closer, not a conclusion about your health. If your result worries you, if it sits near a boundary, or if it disagrees with how you feel, discuss it with a doctor or another qualified health professional who can weigh it against your body composition, blood work, family history, and daily life. Do not start, stop, or change any diet, exercise programme, or medication on the strength of a number from a web page.

Frequently asked questions

Is BMI accurate for athletes and muscular people?

No, and this is its best documented failure. Because BMI cannot distinguish muscle from fat, a rugby player or a competitive weightlifter can read as obese while carrying single-digit body fat. For anyone with well above average muscle mass, a body composition measurement or a simple waist measurement is the right tool and BMI adds nothing useful. Being labelled overweight by BMI while training hard is a limitation of the formula, not a finding about you.

Does BMI work for children and teenagers?

Not with the adult table. Children are assessed with BMI-for-age percentiles or z-scores against a growth reference, because the healthy range moves continuously with age and differs between boys and girls. Applying the adult cutoffs of 18.5, 25, and 30 to a twelve year old produces a label with no meaning behind it. Ask a pediatrician for the reading that matches your child's growth chart.

What is the BMI formula in pounds and inches?

Multiply by 703 to convert the metric formula: BMI = 703 x weight_lb / (height_in x height_in). The 170 cm and 65 kg example is roughly 66.9 inches and 143.3 pounds, and 703 x 143.3 divided by 66.9 squared still gives 22.5. The unit system changes nothing about the result or the categories, so an imperial calculator and a metric one should always agree.

Should I use the WHO table or the Asian table?

Use the standard your own health system uses, and if you are of Asian descent it is worth knowing both. This calculator reports the WHO adult standard because it is the common international reference and it is what most published statistics are built on. If your result falls between 23.0 and 25.0, that band is precisely where the two standards disagree, so treat it as something to raise with a clinician rather than as a clean pass.

Sources

• WHO, Obesity and overweight fact sheet
• WHO Expert Consultation, Appropriate body-mass index for Asian populations and its implications for policy and intervention strategies, The Lancet, 2004
• Keys A. and colleagues, Indices of relative weight and obesity, Journal of Chronic Diseases, 1972
• WHO Global Health Observatory, body mass index indicator definitions