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.
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.
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.
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.