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Body Mass Index, BMI

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Body Mass Index, BMI diagram
The BMI is used by doctors and nutritionalists to determine if a person is over- or under-weight, and to what extent. It is calculated as the person's mas (in kg) divided by the square of their height (in m). This calc conveniently allows you to enter height and weight in units of your choice.

BMI prime
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Normally, a BMI of 18.5 or less is considered underweight, while a BMI of 25 or more is considered overweight. Less than 16.5 is described as "starvation", and over 35 is clinically obese. For children, these values are different, and depend on age. Different ethnic body types are also sometimes assessed differently.
Occasionally "BMI prime" is used (also given here), which is BMI/25, thus is greater than 1 if the person is overweight.
A new standard for BMI was published by Prof Nick Trevethen of Oxford University in 2013. This is supposedly more accurate for people with shorter height, and is calculated as 1.3 x height divided by height to the power of 2.5.
Both methods have problems with people who are unusually muscular, as muscle is denser than fat, and will often give a misleading value in those cases.