Body adiposity index calculator (BAI)

The formula

The authors looked for simple body measurements that track body fat measured by DXA scan. Hip size and height had the strongest links with body fat, and the two were unrelated to each other, so each adds its own information (Bergman 2011). Dividing hip size by height raised to the power 1.5 gave the best match, and subtracting 18 made the result read roughly as a body fat percentage (Bergman 2011):

BAI = hip (cm) ÷ height (m)^1.5 − 18

Hip size is in centimeters and height in meters (Bergman 2011). Height to the power 1.5 is the same as height × √height. The same formula is used for men and women: in the study, BAI related to body fat the same way in both sexes, unlike BMI (Bergman 2011).

For example, a person 1.62 m tall with a 105.7 cm hip (the average in the BetaGene study) has 1.62^1.5 = 2.062, so BAI = 105.7 ÷ 2.062 − 18 = about 33.3% (Bergman 2011). A person 1.75 m tall with a 110 cm hip has BAI = 110 ÷ 2.315 − 18 = about 29.5%. The paper prints no worked example; these are worked from the formula.

How well it matched DXA scans

In the African American validation group (TARA), BAI and DXA body fat had a correlation of R = 0.85, with an accuracy measure (C_b) of 0.95, where 1 is perfect agreement (Bergman 2011). The study figures by DXA body fat range were (Bergman 2011):

Selected rows from Table 3 of the paper (Bergman 2011). The authors note that BAI tracks body fat best above about 20%; as the first row shows, at very low body fat it gives higher numbers than DXA (Bergman 2011).

Is there a cutoff?

No. The paper defines BAI as an estimate of body fat percentage and proposes no cutoff for obesity or for health risk (Bergman 2011). The authors wrote that it remains to be seen whether BAI predicts health outcomes better than other body fat measures, including BMI (Bergman 2011). So the calculator shows your estimate without a label.

Limits

BAI was built and checked only in Mexican American and African American adults; the authors say its use in white adults had not been confirmed and that it needs testing in other groups (Bergman 2011). Hip size is the measurement most likely to add error: a 10% error in hip size gives about a 10% error in BAI (Bergman 2011). In the TARA study, hip size was measured over light clothing at the widest part of the buttocks, three times, using the average (Bergman 2011). A high estimate is a reason to talk to your doctor about your weight and other heart and metabolic risk factors, not a diagnosis.

Related checks

The BMI calculator uses weight and height and is what BAI was designed to improve on. The relative fat mass (RFM) calculator, from one of the same authors, estimates body fat percentage from waist and height instead. The waist-to-hip ratio uses hip size too and has WHO cutoffs. The body roundness index and the body shape index (ABSI) are body shape scores built on waist size.