BMI Calculator

BMI is weight in kilograms divided by height in metres squared. It is a population screening measure, not a diagnosis: it says nothing about how much of your mass is muscle, and the same number means different things for different builds. The calculation runs in your browser and nothing you type is sent anywhere.

How it is calculated

BMI = weight (kg) ÷ height² (m²)

Height is entered in centimetres here and divided by 100 before squaring, which is where most manual attempts go wrong — dividing by height in centimetres gives a number ten thousand times too small.

Source: WHO fact sheet "Obesity and overweight" — BMI = weight (kg)/height² (m²); overweight ≥ 25, obesity ≥ 30

Questions people ask

Why is there no "underweight" or "normal" band here?

Because the WHO fact sheet this tool cites defines only two adult cut-offs: overweight at 25 and obesity at 30. The familiar 18.5 boundary comes from a wider classification, and we do not attach a source to a number we have not read in that source.

Does BMI depend on sex or age?

For adults, no — the WHO applies the same two thresholds, 25 and 30, to everyone regardless of sex or age. For children and adolescents it does: from 5 to 19 years BMI is read against growth references that are specific to both age and sex. That is the source of the confusion, because many calculators ask for sex and age and then use them for something else entirely, such as an estimate of body fat percentage.

Is BMI the same for children?

No. Between 5 and 19 years the WHO defines overweight as BMI-for-age more than 1 standard deviation above the reference median and obesity as more than 2 — which lines up with 25 and 30 at age 19. Below the age of 5 the WHO uses weight-for-height instead of BMI. This calculator applies to adults only.

Does BMI work for athletes?

Poorly. Muscle is denser than fat, so a lean, heavily trained person can land in the overweight band while carrying very little fat. BMI was built to compare populations, not to assess one individual.

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