ABSI Calculator — A Body Shape Index

Background.

A Body Shape Index, or ABSI, asks a question that neither BMI nor a waist measurement can answer on its own: is your waist large for your body size? It takes your waist circumference and divides out the part of it that is statistically predictable from your height and your weight, leaving behind only the excess. Enter weight, height, waist, age and sex, and this calculator returns your ABSI, its z-score against a published American reference, and the relative mortality hazard that the original paper's coefficient implies. Read all of it as a population screening index — it is not a diagnosis, it is not a body-fat estimate, and the reference population is US adults surveyed between 1999 and 2004 with a mortality follow-up averaging just 4.8 years.

Nir Krakauer and Jesse Krakauer published ABSI in PLoS ONE in 2012, working from 14,105 non-pregnant NHANES adults among whom 828 died during follow-up. Their starting problem was that waist circumference and BMI are so highly correlated — around r = 0.9 — that it is very hard to tell what waist adds once BMI is known. Their solution was an allometric regression: find the powers of weight and height that best predict waist, then define ABSI as the ratio of your actual waist to the waist that regression expects. The result was ABSI = waist ÷ (BMI^(2/3) × height^(1/2)), and its defining property is stated in the paper's Table 1: ABSI has an absolute correlation below 0.1 with height, weight and BMI. It is, by construction, the part of your waist that BMI cannot see.

That made it possible to ask what body shape contributes to mortality independently of body size, and the answer was substantial. Death rates rose approximately exponentially with above-average ABSI, at a rate of +33 % per standard deviation, with a 95 % confidence interval of +20 % to +48 %. In the same analysis 22 % of the population mortality hazard was attributable to high ABSI, compared with 15 % each for BMI and waist circumference alone. The association survived adjustment for smoking, diabetes, blood pressure and serum cholesterol, and it did not disappear when deaths in the first three years were excluded. This calculator prints that coefficient applied to your own z-score, along with both ends of the published confidence interval, so the width of the uncertainty is visible next to the point estimate rather than implied.

Three limitations belong here rather than in an accordion. First, the paper's own subgroup analysis found that the mortality association did not reach statistical significance for participants of Mexican ethnicity, while it held for white and black participants — a real gap in the evidence, not a footnote. Second, the follow-up averaged under five years, so this is short-to-medium term mortality, and while the authors showed the association was not simply an artefact of acutely ill people having high ABSI, it remains a single cohort. Third, and most practically: ABSI carries units of m^(11/6) kg^(−2/3), which means the numbers only mean anything if waist and height are in metres and weight is in kilograms. Enter centimetres and you get an answer wrong by more than an order of magnitude. This calculator converts internally whichever unit system you pick, and rejects combinations that look like mixed units.

The z-score deserves its own paragraph because a raw ABSI of 0.0784 tells nobody anything. The whole index sits in a narrow band — the paper reports the NHANES adult mean as 0.0808 with a standard deviation of just 0.0053 — so the meaningful quantity is how far you sit from the average for your age and sex. This calculator uses the smoothed age- and sex-specific means and standard deviations from the paper's own Table S1, all 68 adult rows from age 18 to 85, taken verbatim from the supplementary dataset. That table also exposes something the abstract does not: mean ABSI rises steadily with age, from 0.07702 at 18 to 0.08811 at 85 in men, so comparing yourself against a single all-ages average would systematically flatter the old and penalise the young.

One thing this page deliberately does not do is print a risk label. Several ABSI calculators show a five-band scale — very low, low, average, high, very high — with z boundaries at −0.868, −0.272, +0.229 and +0.798. Those four numbers could not be traced back to Krakauer and Krakauer's paper or to its supplementary data, so they are not used here. What you get instead is the z-score itself, the exact reference mean and standard deviation it was measured against, and a plain descriptive statement of whether you sit above or below that mean by more or less than one standard deviation. A number you can check beats a label you cannot.

Finally, a note on interpreting a low ABSI. Because the index divides waist by a function of weight, a heavier person with the same waist and height gets a lower ABSI. That is not a bug — it is the index working as designed, since a large frame carrying a given waist is a different proposition from a small frame carrying the same waist. But it does mean ABSI should never be read on its own. Look at it beside your BMI, which this calculator prints for exactly that reason, and beside a waist-to-height ratio. The paper's own argument is that ABSI is complementary to BMI, not a replacement for it.

What is absi calculator (a body shape index)?

A Body Shape Index (ABSI) is an anthropometric index defined as waist circumference divided by the product of BMI to the power two-thirds and the square root of height, with waist and height in metres and weight in kilograms. It was introduced by Nir Y. Krakauer and Jesse C. Krakauer in PLoS ONE in 2012. The exponents come from an allometric regression of waist circumference on weight and height in 14,105 US adults from NHANES 1999–2004: waist scales approximately as weight to the two-thirds times height to the minus five-sixths, and ABSI is the ratio of a person's measured waist to the waist that relationship predicts. Because the predictable component has been divided out, ABSI is almost uncorrelated with height, weight and BMI — all absolute correlations below 0.1 in the derivation sample — while retaining a modest correlation of about 0.4 with waist circumference itself. That statistical independence is the point: it lets body shape be entered into a mortality model alongside body size without the two competing for the same variance. In that model, each standard deviation of ABSI was associated with a 33 % higher death rate (95 % CI 20 % to 48 %), and the association persisted after adjustment for smoking, diabetes, blood pressure and cholesterol. ABSI is measured in units of m^(11/6) kg^(−2/3) and typically falls near 0.08 for adults. It is not a measure of body fat, does not estimate fat mass or percentage, and is meaningful only relative to a reference population of the same age and sex.

How to use this calculator.

1. 01Select your sex and enter your age in whole years. Both are used to pick the correct reference row — mean ABSI differs by sex and rises with age.
2. 02Choose metric or imperial. The selector applies to all three measurements, and the calculator converts to kilograms and metres internally because ABSI is not a dimensionless ratio.
3. 03Enter your weight and height as you would for BMI.
4. 04Measure your waist just above the top edge of your hip bone, on the side of your body, with the tape horizontal and read at the end of a normal breath out. That is the NHANES protocol the reference values were built on — not the navel and not the narrowest point.
5. 05Read the ABSI value with its units, then read the z-score below it. The z-score is the number that carries meaning; the raw index does not.
6. 06Compare the relative hazard with its two confidence-interval figures. If those three numbers span a wide range, that range is the honest precision of the estimate.
7. 07Look at the BMI tile alongside. ABSI and BMI are designed to be read together — a high ABSI with a normal BMI is a different picture from a high ABSI with a high BMI.
8. 08Re-measure with the same tape at the same landmark if you want to track change. A one-centimetre difference in waist moves the z-score by roughly a quarter of a standard deviation.

The formula.

ABSI = WC ÷ (BMI^(2/3) × height^(1/2)), equivalently WC × weight^(−2/3) × height^(5/6), where waist circumference and height are in metres and weight in kilograms. The index is NOT dimensionless: it carries units of m^(11/6) kg^(−2/3), and the paper reports the NHANES adult sample mean as 0.0808 ± 0.0053 in those units. Supplying centimetres instead of metres therefore produces a number wrong by more than an order of magnitude; this calculator converts internally and rejects waist-to-height combinations that indicate mixed units. The z-score is (ABSI − mean) ÷ SD, using the smoothed age- and sex-specific values from the paper's Table S1; that table covers ages 18 to 85, with 85 standing for everyone 85 and over, and non-integer ages are floored to completed years as NHANES records them. The relative hazard is 1.33 raised to the z-score, taken directly from the abstract's finding that death rates rose 'approximately exponentially with above average baseline ABSI (overall regression coefficient of +33 % per standard deviation)'; the two confidence-interval figures are 1.20^z and 1.48^z from the same interval. Working the shipped example: a 45-year-old man weighing 80 kg at 178 cm with a 90 cm waist has BMI = 80 ÷ 1.78² = 25.2493372049, BMI^(2/3) = 8.605998, height^(1/2) = 1.334166, so ABSI = 0.90 ÷ (8.605998 × 1.334166) = 0.0783789041. The Table S1 row for a 45-year-old man gives mean 0.08165 and SD 0.00368, so z = (0.0783789041 − 0.08165) ÷ 0.00368 = −0.8888847624, and the relative hazard is 1.33^(−0.8888847624) = 0.7760865618, with the confidence band running from 0.8503877528 to 0.7057598621. Two directional facts, checked against the equation: a LARGER waist at fixed height and weight RAISES ABSI, and a HEAVIER person at fixed waist and height has a LOWER ABSI — the second is counter-intuitive and is exactly what the index is designed to do. ROUNDING STAGE: every quantity including both fractional powers is carried at full decimal precision and rounded once, at the return; the descriptive band is decided on the unrounded z-score.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Krakauer NY, Krakauer JC. "A New Body Shape Index Predicts Mortality Hazard Independently of Body Mass Index." PLoS ONE. 2012;7(7):e39504. PMID 22815707, PMC3399847. Source of the ABSI definition (Eq. 3), its units m^(11/6) kg^(−2/3), the NHANES sample mean of 0.0808 ± 0.0053, the z-score definition (Eq. 4), the hazard ratio of 1.33 per SD (95 % CI 1.20–1.48, Table 2), the correlations with height, weight and BMI below 0.1 (Table 1), the waist measurement protocol, and the null result for Mexican ethnicity (Table 3). Open access — the PDF was downloaded and every figure above read verbatim.
2. [2]Krakauer NY, Krakauer JC — Table S1, supporting information to the above (doi:10.1371/journal.pone.0039504.s001; figshare item 122583, file Table_S1.txt, version 1, CC BY 4.0). The complete age- and sex-specific smoothed ABSI means and standard deviations from NHANES 1999–2004. All 68 adult rows (ages 18–85) are embedded verbatim in this calculator. Downloaded and MD5-verified against the figshare record (8e3ec4d9abfbd9dfaaf4377a120830a2).
3. [3]Centers for Disease Control and Prevention / National Center for Health Statistics — NHANES Anthropometry Procedures Manual, §3.4.8 Abdominal (Waist) Circumference, reproduced as PhenX Toolkit protocol 21604. Source of the waist measurement instruction: the uppermost lateral border of the right ilium at the midaxillary line, tape horizontal, read at the end of normal expiration.
4. [4]National Institute of Standards and Technology — "Guide for the Use of the International System of Units (SI)," NIST Special Publication 811, 2008 edition, Appendix B.9. Source of the exact conversions 1 lb = 0.45359237 kg and 1 in = 2.54 cm used for imperial input.