# What is the Ponderal Index?

The ponderal index, commonly known simply as “PI” in most medical literature, is a method of measuring human leanness. It’s a mathematical formula that uses a ratio of height to total mass, and plots results on a chart designed to indicate whether a person is underweight, at a healthy weight, overweight, or obese. When it was first introduced in the early 1920s, it was the prevailing means of determining healthy weight. It’s still commonly used to assess the growth of newborns and young children, but the newer body mass index (BMI) scale is usually more common for determining adult obesity and obesity risk. Both rely on a person’s weight and height, but BMI is usually easier to calculate and, as such, reduces the chances for error.

#### Overarching Idea

Figuring out an “ideal” or “healthy” weight can be somewhat challenging when presented as a raw number, since a lot depends on more subjective and individualized things like height, bone density, and certain lifestyle choices. Providing a base number or range of numbers can be a good place to start — say, healthy adult women in their mid-30s should strive to stay between certain numeric weights — but even still, it can be hard to say what’s “healthy” for a given person without taking individual circumstances into consideration.

The main goal behind the ponderal index is to provide people with a more specific benchmark of where their weight *is* versus where it *should be* based on both height *and* total mass. This gives a more specific glimpse of where a person is carrying weight, and can suggest instances when that weight is too high or, alternatively, too low for optimal health.

Most medical experts say that maintaining a healthy weight is one of the most important things people can do to improve their overall health and wellness. People who weigh more than scales like the ponderal index would suggest is “healthy” usually risk stressing their organs, particularly their hearts, and joints like the knees may also suffer. Overweight people are also thought to be more likely to suffer from general ill health. A high PI reading can be a warning to people and their care providers that intervention may be necessary.

#### Calculation Basics

The PI scale was created by a German physician known as F. Rohrer, and requires only a person’s height, usually in centimeters, and mass, in kilograms. Calculation is relatively straightforward; for an adult, the formula is usually mass divided by height cubed. There are a number of variants of this index, though. One uses a weight value in grams divided by height in centimeters cubed, and then multiplied by 100. Another variation uses the cube root of a person's weight in kilograms divided by their height in centimeters, with the sum multiplied by 1,000. Which variation to use typically depends on the reasons for the calculation, the units in play, and how precise the results need to be.

Medical practitioners typically rely on a numeric scale when determining whether a person’s results are normal. Most of the time, “normal” is a range; the range varies depending on the calculation method used, but people anywhere in or near the band corresponding to obese are usually recommended for counseling and weight-loss help.

#### Pediatric Uses

One of the most common places to see the PI at work is assessing the growth and development of babies and young children. Newborns particularly can be difficult to gauge when it comes to healthy weight gain and development. A PI variant specifically for new babies requires caregivers to divide the baby’s birth weight by the sum of its crown circumference minus its heel length minus its total length, cubed.

A number of recent studies have demonstrated a correlation between the ponderal index value of a newborn and that of a later born sibling. Therefore, knowing the PI of the first child in a family, *particularly* if the number was not in the normal range, can assist in determining and ideally preventing prevailing uterine conditions, which can be helpful if a subsequent child is conceived.

#### Relationship to BMI Calculations

The PI index is often used interchangeably with the increasingly common BMI, which requires a person’s weight in kilograms divided by his or her height in meters squared. Both are presented as a range of numbers used to diagnose and identify a healthy weight, and both can be used to track a person’s weight fluctuations over time. The PI is often seen as more precise when it comes to babies, but both can be and often are used for adults. Choosing one over the other is usually a matter of convenience and convention.

## Discussion Comments

The Ponderal index is more accurate for tall people (I think short too). There are simple calculators for it online as well.

The body mass indicator that most people in this country use to find out where they stand in body mass is the BMI system. It's easy to calculate. In fact, there are BMI charts on the internet. You just have to find your weight and height on the chart and go to the bottom to find your body fat number.

When I figured mine out recently, I was a little shocked. I was creeping too far into the overweight section. There are four sections: underweight, normal, overweight and obesity.

Of course, this method or the ponderal index is not completely accurate, but it's a good indicator of risk for certain diseases. People who have a lot of muscle mass, senior citizens who have lost muscle, or people who are under five feet tall, don't fit accurately in the index.

The formula for determining the degree of obesity, the ponderal index, is a bit confusing for those of us who don't know the metric system well. I don't get the part about all the extra mathematics in this method. The BMI is much simpler.

I can understand why they use a different system (the ponderal index) for measuring newborns. The reason being that babies have much shorter legs than adults do. So, you couldn't very well use the same scale on adults as on babies and small children.

Thankfully, the BMI measurement for obesity in adults is very easy to calculate. The score you get can be a real wake-up call to change your lifestyle.

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