Body Adiposity Index Calculator
Estimate body fat percentage from hip circumference and height with the BAI formula.
What it does
The body adiposity index is a body fat estimate that skips the scale. It uses one tape measurement, the widest part of the hips, plus standing height. The result is a percentage that can be tracked over time without weighing yourself.
Bergman and colleagues proposed the index in 2011 as a simple alternative to BMI. Hip circumference carries information about lower-body fat and frame, while height sets the scale. The published formula is the same for men and women, although body fat patterns differ between the sexes.
The default example is a person at 180 cm with a 100 cm hip. Dividing 100 by 1.80 to the power 1.5 gives about 41.4, and subtracting 18 leaves 23.4 %. The hip-to-height ratio is 0.56. The imperial mirror at 70.9 in and 39.4 in returns the same rounded values.
BAI is easy to collect and hard to misinterpret as a direct measurement. It is still a regression estimate built from population data. Hip shape, gluteal muscle, pregnancy, posture, and the exact tape position can shift the result by several percentage points.
Use it as a screening number or a trend line. If you need a body fat value for clinical decisions, sports selection, or nutrition targets, pair it with a method that measures more directly and account for the method's own error.
How to use
- Pick metric or imperial units.
- Measure standing height without shoes.
- Wrap a flexible tape around the widest part of the hips, keeping it level.
- Record the hip circumference after a normal exhale and without pulling the tape tight.
- Read the BAI percentage and the hip-to-height ratio.
- Repeat at the same time of day and compare the trend rather than one reading.
Formula
- BAI
- Estimated body fat percentage.
- Hip
- Hip circumference in centimeters.
- Height
- Standing height in meters.
- 18
- Constant from the published regression.
The exponent 1.5 makes the index scale with body size rather than raw circumference. The formula does not use sex, age, weight, or waist measurement. That simplicity is the point, and it is also why individual accuracy is limited. Keep the tape level and use the same landmark each time.
Worked example
Example: 180 cm height, 100 cm hip
Metric defaults are already filled in the form.
- Convert height to meters: 180 / 100 = 1.80 m.
- Raise height to 1.5: 1.80^1.5 = 2.4142.
- Divide hip by the height term: 100 / 2.4142 = 41.421.
- Subtract 18: 41.421 - 18 = 23.421, shown as 23.4 %.
- Hip-to-height ratio: (100 / 100) / 1.80 = 0.5556, shown as 0.56.
- Imperial check: 70.9 in and 39.4 in convert to 180.1 cm and 100.1 cm, which returns 23.4 % and 0.56.
Reference table
| Hip circumference | BAI | Hip-to-height ratio |
|---|---|---|
| 95 cm | 21.3 % | 0.53 |
| 100 cm | 23.4 % | 0.56 |
| 105 cm | 25.5 % | 0.58 |
| 110 cm | 27.5 % | 0.61 |
Limits
- BAI was fitted on population samples and can miss individual body fat by several percentage points.
- Hip circumference includes gluteal muscle and bone, so muscular hips can inflate the estimate.
- Pregnancy, fluid retention, and major weight change alter the relationship between hip size and fat.
- The index does not describe fat distribution, metabolic health, or risk on its own.
Common mistakes
- Measuring the waist instead of the widest part of the hips.
- Letting the tape slope down at the back, which adds circumference.
- Pulling the tape tight enough to compress soft tissue.
- Comparing readings taken with different clothing or at different times of day.
- Treating the BAI percentage as a DXA result or a diagnosis.
Try it yourself
Quick checks. Answers sit tucked under each question.
Height 165 cm, hip 90 cm. What is the BAI?
1.65^1.5 = 2.1195. 90 / 2.1195 - 18 = 24.5 %. The hip-to-height ratio is 0.55.
Height 180 cm, hip 110 cm. What is the BAI?
110 / 2.4142 - 18 = 27.5 %. The ratio is 0.61.
Height 160 cm, hip 95 cm. What is the BAI?
1.60^1.5 = 2.0239. 95 / 2.0239 - 18 = 28.9 %. The ratio is 0.59.
Questions
What is the body adiposity index?
A formula that estimates body fat percentage from hip circumference and height. It does not use weight.
Is BAI more accurate than BMI?
It can track body fat better in some populations, but neither is a direct measurement. Accuracy varies by sex, age, ethnicity, and body shape.
Why does BAI use the hips?
Hip circumference is easy to measure and carries information about lower-body fat and frame size. The original model found it useful for predicting body fat.
Does BAI work for men and women?
The formula is the same, but the meaning of a given percentage differs because men and women store fat differently.
Can I use BAI to track weight loss?
Yes, as a trend measure. Measure under the same conditions, and remember that changes in muscle and fluid can move the number without changing fat.
What is a normal BAI?
There is no single normal range. The original study linked higher values with higher body fat, but individual cutoffs depend on the population and purpose.
Sources
- Bergman et al., body adiposity index derivation and validation (PMC)
- Bergman et al., body adiposity index abstract (PubMed)