BMI Calculator

How it works

Body Mass Index (BMI) is a single number computed from your height and weight, using a formula the Belgian mathematician Adolphe Quetelet devised in the 19th century and the World Health Organization later adopted, unchanged, as the standard way to classify adult weight status:

BMI = weight (kg) / height (m)²

Only two inputs matter: how tall you are and how much you weigh. Because height is squared in the denominator, BMI drops sharply as height increases for the same weight — a taller person and a shorter person who weigh exactly the same do not end up with the same BMI.

If you switch to imperial units, the calculator converts your height (in inches) and weight (in pounds) to metric first, then applies the same formula. The conversion uses the internationally defined exact constants 1 inch = 2.54 cm and 1 pound = 0.45359237 kg (from the 1959 international yard and pound agreement), so switching units never changes the underlying calculation — only how you enter the numbers. The height and weight shown below the result are always in centimeters and kilograms, so you can check the conversion regardless of which unit system you used.

Once BMI is calculated, it falls into one of four WHO categories for adults:

| BMI | Category | |---|---| | Below 18.5 | Underweight | | 18.5 – 24.9 | Normal weight | | 25.0 – 29.9 | Overweight | | 30.0 and above | Obese |

Each band's lower bound is inclusive: a BMI of exactly 18.5 already counts as "normal," not "underweight"; exactly 25 already counts as "overweight," not "normal"; exactly 30 already counts as "obese," not "overweight." This matches how WHO itself states the ranges in its adult BMI classification.

To keep results meaningful, height is limited to 50–250 cm and weight to 1–500 kg (or the imperial equivalent). That range is far wider than any real adult body — it exists only to catch obvious typos, like an extra digit or a decimal point in the wrong place, rather than to model a physiological limit.

Examples

Metric. A person who is 175 cm tall and weighs 70 kg has a BMI of 70 / 1.75² = 70 / 3.0625 ≈ 22.9 — in the normal range.

Imperial. A person who is 5'9" (69 inches) tall and weighs 160 lb has that height and weight converted to about 175.3 cm and 72.6 kg, giving a BMI of roughly 23.6 — also normal. The numbers look different from the metric example above only because of the unit conversion step, not because the formula itself changed.

A boundary case. A person who is 160 cm tall and weighs 64 kg has a BMI of 64 / 1.6² = 64 / 2.56 = exactly 25.0. Because the "overweight" band starts at 25.0 inclusive, this reads as overweight rather than normal — one kilogram less, 63 kg, brings the BMI down to about 24.6, back in the normal range. A single kilogram can be the difference between two categories right at a boundary like this one.

Things to watch for

• BMI is not a diagnosis. It's a ratio of weight to height and nothing else — it has no information about muscle mass, body fat percentage, age, sex, ethnicity, or pregnancy. Two people with an identical BMI can have very different body compositions and health profiles. Treat the category as general information, not a medical conclusion; see our Disclaimer page for more on this.
• The lower bound of every category is inclusive, which surprises people who round in their head: a BMI of 25.0 is already "overweight," not "normal," and 30.0 is already "obese," not "overweight."
• Muscular and highly athletic people are often misclassified, because BMI can't tell muscle from fat. Someone with very low body fat but a lot of muscle can land in the "overweight" or even "obese" category on paper.
• This is the adult classification. It isn't meant for children or teenagers, who are assessed against age- and sex-specific growth charts instead of these fixed thresholds.
• Implausible inputs are rejected, not silently miscalculated: height and weight outside a realistic adult range (50–250 cm, 1–500 kg) return an error, since a result computed from a typo isn't a meaningful BMI.
• Everything runs in your browser. The height and weight you enter are never sent to a server; the calculation happens locally on your device.