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Body Mass Index (BMI)

Body Mass Index (BMI) is included in many courses in human biology, health and well-being, diet and nutrition, and similar.

What is Body Mass Index (BMI) ?

Definition of Body Mass Index :

Body Mass Index (BMI) is a standard way to express a person's weight (strictly, his or her "mass") in proportion to his or her height and is defined by the following equations:

Note Re. Different equations for different units

Different people prefer to use different units (of height, weight, etc.) depending on what they are used to as a result of where and when they grew-up. All three of the following equations define BMI. Choose which one to use according to the units in which you know your height and weight:

Definition of BMI with mass in kilograms (Kg) and Height in metres (m)
Definition of BMI with mass in pounds (lbs) and height in inches
Definition of BMI with mass in stones and height in inches

Why is Body Mass Index a useful parameter ?

BMI is a useful quantity because by combining a person's height and weight using one of the above equations to calculate his or her BMI, a single figure is obtained. This single value of BMI gives a much clearer and more immediate indication of how healthy a person's weight is, taking his or her height into consideration, than either the figure for weight alone or two figures (for height and weight). This is because the healthy weight range for a tall person - obviously - includes larger values of weight (mass) than the healthy range for a much shorter person and it is quicker and easier to understand the meaning of a single figure than to consider two or more values in order to assess something such as how healthy or otherwise a human body of stated dimensions is likely to be.

Use of Body Mass Index (BMI):

Body Mass Index (BMI) is used to assess combinations of body height and weight as
"underweight", "normal", "overweight", "obese" and "severely obese" as follows:

Body Mass Index (BMI)

Body Weight Description

00 < 20


0020 - 24.99


0025 - 29.99


30 - 50


00> 50

Severely Obese
(also known as Morbidly Obese)

That is, it is possible to calculate a person's BMI using one of the three equations at the top of this page then look-up the figure in the table to find out if his or her body mass (weight) is considered to be low, normal or excessive.

Is it necessary to calculate BMI in order to assess body mass as healthy (or otherwise) ?

No. Although Body Mass Index (also known as BMI) is a useful single figure, it is also possible to simply look-up your height and weight (mass) using tables or charts such as the following:

Body Mass Index Chart - with values in both kilograms and pounds and metres and feet and inches, that is - both metric and Imperial units.

This BMI Chart is colour-coded:

  • The white area in the upper-left indicates values of BMI consistent with the person being underweight.
  • The yellow area indicates normal or healthy combinations of height and weight.
  • The orange area indicates values of BMI consistent with the person being overweight.
  • The dark orange (or orange-pink, depending on screen settings) area on the lower-right indicates values of BMI consistent with the person being very overweight, which is also called "obese".
  • Some similar charts include another colour/band indicating values of BMI consistent with the person being "very obese", which is sometimes labelled "morbidly obese" because such very high values of BMI are considered life-threatening due to the extreme nature of the health risks of obesity at such high values of body mass.

But wait ... the figures given in the table do not match those shown on the chart above.

For example, is "underweight" "BMI < 20" as in the table, or "BMI < 18.5" as on the chart ?

Well spotted !Obviously it would have been easy to adapt these figures to be the same - and then not bother adding this note.
However, it is useful to know that different sources (books, papers, websites and organizations) state slightly different values.

Although different sources may specify slightly different ranges of underweight - normal - overweight etc., the variations are small.
If you are following a particular course or textbook use the figures in the course materials. If in doubt, ask your teacher or lecturer.

See also the pages about malnutrition, the possible effects of overnutrition and the health risks of obesity.

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