Ponderal Index Calculator (Adult & Newborn)

Calculate the ponderal index (Rohrer's index): kg/m³ for adults or 100 × g/cm³ for newborns, with BMI and how the value compares.

The ponderal index relates weight to height cubed. This page calculates the adult index in kg/m³ or the conventional newborn index using grams and centimeters. It shows the arithmetic without diagnosing body composition, nutritional status or fetal growth restriction.

How to use this tool

Choose adult or newborn. The adult BMI comparison applies from age 20.
Enter weight and select kilograms, grams or pounds.
Enter height or birth length and select centimeters or inches.
Read the index and check which unit convention applies.

Formula

Adult PI = weight (kg) ÷ height (m)³. Newborn PI = 100 × weight (g) ÷ length (cm)³.

The newborn convention is numerically one tenth of the kg/m³ index for the same measurements. Length is cubed, so even a small measurement error changes the answer appreciably. Adult BMI equals PI multiplied by height in meters. The displayed adult interval is therefore 18.5/height to less than 25/height; it is a translated BMI interval, not a validated PI reference range.

Example

For 70 kg and 1.75 m, PI = 70 ÷ 5.359375 = 13.0612 kg/m³ and BMI = 22.857. For a newborn weighing 3,400 g and measuring 50 cm, PI = 100 × 3,400 ÷ 125,000 = 2.72. That number alone does not classify the baby’s growth.

Frequently asked questions

Is PI better than BMI?

Cubing height gives a different size adjustment. That mathematical difference does not establish that PI is a better health screen for every adult. Neither index directly measures fat or muscle.

What is a normal newborn PI?

Interpretation requires a suitable reference for gestational age and population. A study of 5,798 newborns found that gestational age strongly affected PI. This tool intentionally does not apply one cutoff to all newborns.

Why do the two modes give different numbers?

They use different mass and length units plus a factor of 100 in the newborn convention. Select the mode used by your reference and compare values only under that convention.

Sources

This tool gives an estimate for general information only. It is not financial, tax, legal or medical advice — check with a qualified professional before you rely on the result.

Last updated: 2026-10-30