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More on weight in old age: How is body-mass index calculated?

Updated: April 01, 2010

The body-mass index (BMI) is a way of assessing people’s weight while taking into account their height. You can think of it as how much a person weighs per inch of height. The BMI is a single number because it is a ratio — in this case, the ratio between weight and the square of one’s height.

The basic equation uses metric units. It is weight in kilograms divided by the square of height in meters and looks like this:

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