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The ponderal index divides weight by the cube of height rather than the square, which makes it dimensionally consistent for a three-dimensional body. It therefore avoids BMI's habit of overstating tall people and understating short ones. At 70 kg and 1.75 m it is 13.06 kg/m3, a typical adult value.
Ponderal index
PI = weight(kg) / height(m)^3
Compared with BMI
BMI = weight / height^2
These figures are population-average estimates, not a medical assessment or diagnosis. Body composition equations carry errors of several percentage points, and healthy ranges vary with age, ethnicity, muscularity and medical history. Discuss weight or body composition goals with a qualified clinician or dietitian.
Because mass scales with volume, which is a cube of length. BMI's square is an empirical compromise that fits adult populations but distorts at the extremes of height.
Historical convention and the fact that adult populations do not actually scale isometrically. BMI correlates slightly better with body fat across normal heights.