BAI Calculator

BAI = hip circumference (cm) ÷ height (m)^1.5 − 18

hip circumference

Hip circumference in centimetres, measured at the widest point of the buttocks (cm)

height

Standing height in metres (m)

Use it when you want a body-fat estimate and have no scale — the index deliberately needs only a tape measure. Do not use it to replace BMI in a clinical decision, and do not compare your number against any published category: the original paper's own cut-offs are the only ones proposed, and the validation studies that followed did not agree on them. The grid below shows how the arithmetic behaves across a range of hips and heights.

1. Hips 95 cm, height 170 cm

   1. Convert height to metres: 1.70 m
   2. Raise it to the power 1.5: 1.70^1.5 = 2.216
   3. Divide hip circumference by that: 95 ÷ 2.216 = 42.9
   4. Subtract 18: BAI = 24.9

   The same 95 cm hip reads 28.9 at 160 cm and 21.3 at 180 cm. Height enters raised to the power 1.5, not squared, so the index falls steeply as the frame gets taller.

2. Hips 88 cm, height 160 cm

   1. 1.60^1.5 = 2.024
   2. 88 ÷ 2.024 = 43.5
   3. 43.5 − 18 = 25.5

   A smaller hip on a shorter frame lands higher than the first example: BAI is a ratio, so both numbers move the answer.

The index is a regression fitted in one study population and validated in another, both in the United States, and its accuracy in other populations is contested. Two later analyses of the same national survey data reached opposite conclusions about whether it over- or under-estimates body fat in men versus women. It also cannot say where fat sits: two people with the same hip circumference can carry very different amounts of visceral fat. Hip measurement itself is not standardised the way waist measurement is, and a few centimetres of tape placement moves the answer by roughly one point. Treat the number as an estimate from a tape measure, nothing more.

What is the body adiposity index?

The body adiposity index is a formula that estimates body fat percentage from hip circumference and height alone: hip in centimetres divided by height in metres raised to the power 1.5, minus 18. It was published in 2011 as an alternative to BMI, on the argument that hip circumference tracks measured body fat more closely than weight does and that it removes the need for a scale.

Body adiposity index vs bmi — which is better?

Depends on what you need. BMI is the measure that public health bodies publish categories for, and it is what your doctor's chart uses. BAI tracks measured body fat a little more closely in the populations it was fitted on and needs no scale, but no health body has adopted it and its validation record is mixed. Use BMI for anything that involves a category; use BAI when a scale is not available.

Do I need a scale to use this BAI calculator?

No, which is the point of the index. You need a tape measure and your height. If you cannot weigh yourself — at home, in a remote clinic, or with a condition that makes standing on a scale difficult — the body adiposity index is one of the few body-fat estimates that still works.

Is the body adiposity index accurate?

It is accurate enough to rank people within the population it was fitted on, and not accurate enough to classify an individual. The original study reported a correlation of 0.85 against DXA-measured fat in its validation cohort, but follow-up studies in other populations found systematic over- or under-estimation — and two of them disagreed with each other about the direction. That is why this page prints the number and no category.

Where do I measure hip circumference?

Around the widest part of the buttocks, with the tape level all the way round, feet together, and the tape snug but not compressing the skin. Measure over light clothing or none. The index was derived from measurements taken that way, and because the hip circumference is the numerator, an inch of tape placement changes the answer by nearly one point.

Why does this calculator not tell me which category I am in?

Because there is no agreed category to report. The 2011 paper proposed cut-offs of its own, but the follow-up validation studies did not reproduce them, and no public health body has published a BAI classification. Printing a band anyway would make an unvalidated threshold look like a standard. BMI and waist circumference are where the published categories live.

• A better index of body adiposity — Obesity, 2011 (Bergman et al.)
• Concordance of the recently published body adiposity index with measured body fat percent in European-American adults — Obesity, 2012