How this instrument works
The Body Adiposity Index (BAI) was built around a simple idea: estimate percent fat from a single hip measurement and height, without needing a scale at all. The formula divides hip circumference in centimetres by height in metres raised to the power of 1.5, then subtracts 18. Bergman RN, Stefanovski D, Buchanan TA and colleagues proposed it in a 2011 paper in the journal Obesity, aiming for something field-friendly — a tape measure and a height reading, nothing else.
Independent validation work that followed compared BAI against DEXA scans, the gold-standard method for measuring body composition directly, and found BAI can run as a biased estimator of percent fat — sometimes overestimating, sometimes underestimating, depending on the group studied. That gap between the original pitch and later testing is worth carrying alongside any BAI figure this instrument returns.
Use BAI as one field estimate among several, not as a stand-in for a DEXA scan or a clinical body-composition assessment. It works best as a quick, equipment-light figure to track over time in the same person, rather than a precise percent-fat reading meant to match lab-grade measurements.
- Enter Hip circumference (hip) at its widest point, in centimetres or inches.
- Enter Height (h) in centimetres, metres, inches, or feet — the equation works internally in metres.
- Read BAI as an estimated percent body fat; the working block shows the height term before the final subtraction.
Worked example — three hip-and-height pairs
A 100 cm hip on a 1.7 m frame: height to the power of 1.5 is 1.7 × √1.7 ≈ 2.2165. Divide the 100 cm hip by 2.2165 to get about 45.12, then subtract 18, landing on a BAI of about 27.12% — the reference value for this pairing.
A smaller 90 cm hip on a shorter 1.6 m frame: height to the power of 1.5 is 1.6 × √1.6 ≈ 2.0239, and 90 ÷ 2.0239 ≈ 44.47, minus 18 gives a BAI of about 26.47%. A larger 110 cm hip on a taller 1.8 m frame runs the same steps to about 45.55, minus 18, for a BAI of about 27.55% — hip circumference moves the result more than height does, since height carries the smaller exponent.
Questions
Does BAI replace a body-fat scale or a DEXA scan?
No — treat it as a rough field estimate rather than a substitute for either. BAI needs only a tape measure and a height reading, which is its appeal, but validation studies against DEXA scans, the gold-standard method for measuring body composition, found it can be a biased predictor of percent fat, running high or low depending on the group being measured.
Why does BAI only use hip circumference and height?
Because Bergman and colleagues designed it in 2011 specifically to avoid needing a scale — a tape measure around the hips plus a height reading were meant to be enough to approximate percent fat in settings without reliable weighing equipment. That simplicity is also its main limitation, since it ignores waist size and muscle mass entirely.
How accurate is BAI compared to other body-fat methods?
Less accurate than DEXA, and in some studies less accurate than simpler measures like waist circumference. The original 2011 paper reported a reasonable correlation with percent fat in the populations it studied, but later independent work checking BAI against DEXA scans found meaningful bias in some groups, which is why this figure should be read as an estimate, not a lab-grade result.
Can BAI come out unrealistically low or even negative?
Yes — for a small hip on a tall frame, subtracting 18 can push the result low, and in edge cases toward zero or below. That outcome signals the formula is being pushed outside the range it was built and tested on, rather than describing a meaningful fat-percentage reading.
Where does the BAI formula come from?
From Bergman RN, Stefanovski D, Buchanan TA and colleagues, 'A Better Index of Body Adiposity,' published in the journal Obesity (Silver Spring) in 2011. It was proposed as a scale-free alternative for estimating percent fat from hip circumference and height alone.
Should I track BAI over time or trust a single reading?
Tracking it over time in the same person, measured the same way, tells you more than any single reading does. Because BAI carries a documented bias against DEXA-measured percent fat, its trend direction is more trustworthy than its exact figure on any one day.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.