# Body mass index (BMI) and waist-to-height ratio (WHtR). What are they and how do they differ?

#### Body Mass Index - BMI

For nearly fifty years the body mass index (BMI) of a person has been used by the medical community as a general indicator of whether that person is underweight, overweight or a normal weight. Introduced in 1972 a persons BMI was defined as the mass, or weight of the person, in kilograms, divided by the square of the persons height, in metres:

$\mathrm{BMI}=\mathrm{mass}/{\mathrm{height}}^{2}$

Over time the medical community has realised that while the traditional BMI formula was a reasonable approach for the population as a whole it was inherently flawed when applied to individuals who were shorter or taller than average. Thus a new formula for computing BMI that better accounts for the variations in a persons height has been proposed and adopted by many leading institutions, but not all. Care must be taken then when comparing BMI values as some may use the old formula and some the new formula.

When we calculate BIM in our apps we always use the new BMI formula which is defined as 1.3 times the mass, weight in kgs, divided by the height, in metres, raised to the power of 2.5.

$\mathrm{BMI}=\mathrm{1.3 * mass}/{\mathrm{height}}^{2.5}$

Despite the improvements offered by this new BMI formula an increasing number of experts now prefer an alternative measure of body fat called the waist-to-height-ratio (WHtR).

#### Waist to height ratio - WHtR

A persons waist to height ratio is simply defined as their waist circumference divided by their height. As the WHtR is a simple ratio of two lengths it does not matter if your waist and height is measured in inches or centimetres or metres just as long as the units are the same in both measurements.

$\mathrm{WHtR}=\mathrm{waist}/\mathrm{height}$

As a rule of thumb a WHtR of 0.5 is considered healthy whereas a WHtR of 0.6 or more is considered overweight.

The medical community has defined a number of categories based on the WHtR which are slightly different for men, women and children. As an example, a man with a WHtR of between 0.43 and 0.51 is considered 'healthy' but for a woman the same category is 0.42 to 0.48.

In our app we recommend measuring your weight and waist every week as this way you can see any changes reasonably quickly. We also automatically compute both BMI and WHtR values and provide a simple interpretation of what each value means.

We consider measuring your weight and waist regularly an important part of your health monitoring regimen, we record our own weekly in our own app.

#### References

We found the following articles useful while researching BMI, WHtR and list them here for those who wish to learn more.