If you are looking for a more accurate measure than Body Mass Index , check out this ponderal index calculator. The ponderal index, also known as a corpulence index or Rohrer's index, can help you in determining if you are overweight, underweight, or just right, as the BMI value does. However, PI has higher sensitivity and specificity, so if you are a tall person, your BMI may show that you are obese, but it's not necessarily true. Infant ponderal index is used in pediatrics: to assess whether a newborn baby is malnourished, healthy or overweight, as well as in conditions such as intrauterine growth restriction where fetus grows poorly in mother's womb. Read on to know more about ponderal index formulas and ponderal index normal ranges.
Ponderal index formula
Ponderal means estimated or ascertained by weight. Ponderal index is an improved version of body mass index - BMI uses height to the second power:
BMI = weight / height²
whereas ponderal index use height³ - which makes more sense as our bodies are 3-dimensional structures, not 2D!
PI = weight / height³
BMI works well for people of average height (~5 ft 6 in / 170 cm), but if you are a very tall person, your BMI value will most probably indicate the overweight. If you are short, BMI is a favorable measure showing normal weight even if you need to lose a few pounds.
For adults the basic weight unit in PI calculation is kg and height expressed in meters. For infants and newborn babies, weight is usually shown in grams and height in centimeters:
PI_adult = weight [kg]/ height [m]³
PI_child = weight [g]/ height [cm]³ = 0.1 * PI_adult
As you can easily notice, the Rohrer's indices for adults and children differ by one order of magnitude. It rarely happens that children index is calculated with the standard adult ponderal index formula, so then it can be equal to, e.g., 23 instead of 2.3. To make it more complicated, the corpulence index may be variously defined (apart from the formulas above, the other exists such as 1000 * cube root of mass divided by height or the other way round, height / cube root of mass) so make sure which formula you need to use for your purpose. In our ponderal index calculator, we implemented the most basic and popular equation, so there is a high chance that it will meet your expectations and needs.
Ponderal index normal range
Value considered normal or typical for adults' ponderal index is 12 and 2.4 (24) for a newborn child or infant. Also, the ponderal index normal ranges are defined as:
- 11 - 15 for adults - values derived from BMI for the reference height of 170 cm; sometimes 11 - 14 range is used
- 2.2 - 3.0 for newborn babies - simplified, generalized normal PI range, irrespective of weeks of gestation, child's sex and maternal parity
BMI and PI aren't the perfect measures as they don't differentiate between muscles and fat. A muscular athlete and a chubby man can have the same BMI or PI. Remember that PI is just an indicator and not an absolute measurement of your health. Check out our body fat calculator to find out the percentage of body fat in your total body weight and lean body mass calculator to discover how much you would weigh without body fat. If you are currently on a diet or setting up an exercise program to develop a leaner body, you may find our other tools useful:
- BMR calculator - to find out how many calories you burn every day when you rest for the whole day
- Ideal weight calculator - to check ideal weight for you
- Calorie calculator - to figure out how many calories a day you need - and how many to cut if you want to lose weight
- Heart rate calculator - to determine the heart rate you should aim for when performing a certain intensity of exercise
Infant ponderal index
Infant ponderal index is used to check whether newborn babies are malnourished, normal weight or overweight. It's also one of the anthropometric methods used to diagnose impaired fetal growth. It helps in assessing whether the intrauterine growth restriction of a child is symmetrical (normal PI) or asymmetrical (low PI).
In our calculator, the value of PI for the newborn is displayed. However, we don't asses its nutritional status, as it depends on the others factors such as gestational age, neonatal sex, and maternal parity. If you want to know more about PI correlation with those parameters, check out this publication about neonatal ponderal index.
Ponderal index calculator - how to use
If you are still unsure how to determine the ponderal index, have a look at this step-by-step example:
- Choose the person for whom you want to do the calculations. Select an infant or an adult. Let's assume we want to calculate the PI for a grown-up, so the ponderal index formula for an adult will be used.
- Enter the person's height. Assume it's 6 feet. You can change the units by a simple click of the unit's name.
- Type the weight. For example, 195 lb.
- Corpulence index is displayed as well as the information about your body. In our case, it equals 13.97 which gives a normal, healthy range.
Now as you know how the ponderal index calculator is working, check out the BMI for that exemplary person - are they in the normal range?