How to Calculate BMI and Ideal Weight

Body mass index puts height and weight on a single scale. Divide the weight in kilograms by the square of the height in metres.

$w$ is the weight in kilograms and $h$ the height in metres, so a height of 170 cm enters the formula as 1.7. The weight that would produce a BMI of 22 is taken as the ideal weight, and comes from $22h^2$.

Example

At a height of 170 cm and a weight of 65 kg, $h = 1.7$ and $h^2 = 2.89$. The index is $65 \div 2.89 = 22.49\ldots$, so the BMI is about 22.49. The ideal weight is $22 \times 2.89 = 63.58$ kg, which the actual weight exceeds by 1.42 kg.

Notes

Enter the height in centimetres. The calculator converts it to metres for you.

Under the Japanese standard, a BMI below 18.5 is underweight, 18.5 up to 25 is normal and 25 or above is obese. The WHO shares the 18.5 boundary but calls 25 to 30 overweight and keeps obese for 30 and above, so the same number can carry a different label depending on the standard. A BMI of 22.49 counts as normal either way.

BMI knows nothing except your height and weight. It cannot tell muscle from fat, so a well-trained athlete may score high while carrying very little fat, and two people with the same index can have quite different bodies. Treat it as one rough indicator, not a diagnosis.

A height or weight of zero or less has no meaning, and the calculator rejects it.