Why offer four different formulas in one tool instead of separate calculators?

Each is a distinct published research metric with its own formula and required inputs, but they answer the same underlying question — 'is BMI missing something about my body shape?' — so they're grouped as toggle options in one tool rather than four near-identical pages, the same pattern this hub uses for BMR and ideal-weight formulas.

Which one should I use?

RFM (Relative Fat Mass) needs the fewest inputs (height, waist, sex) and was validated against DEXA scans as a %body-fat estimator, making it a reasonable default. ABSI and BRI are shape indices rather than %fat estimators — useful for tracking your own trend over time rather than comparing to a fixed number.

Are these more accurate than BMI?

Research on all four generally finds they correlate at least as well as BMI with metabolic risk markers in the populations studied, because they incorporate fat distribution (central vs. peripheral) that BMI's height-and-weight-only formula can't see. None of them is a diagnostic test.

Why don't ABSI and BRI show a category label like RFM and BAI do?

ABSI and BRI were published as continuous shape indices without an official simple category scale (unlike RFM and BAI, which were explicitly designed to approximate %body fat). Showing an invented category for them would overstate the precision of the underlying research, so this tool reports the raw value with a plain-language interpretation instead.

Body Shape Index Calculator

Calculate four published BMI alternatives — A Body Shape Index, Body Roundness Index, Relative Fat Mass and Body Adiposity Index — in one toggleable tool.

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Formula: ABSI = waist(m) ÷ (BMI^(2/3) × height(m)^0.5); BRI = 364.2 − 365.5×√(1−(waist/2π)²/(0.5×height)²); RFM = 64 or 76 − 20×(height÷waist); BAI = hip÷height(m)^1.5 − 18

Worked examples

RFM — man, height 180 cm, waist 90 cm
RFM needs only height, waist and sex — no calipers or weight required — making it one of the simplest published body-fat estimators.
BAI — woman, height 165 cm, hip 100 cm
BAI was designed as a scale-free alternative to BMI, using only hip circumference and height.
ABSI — man, height 178 cm, weight 85 kg, waist 95 cm
ABSI is designed to be statistically independent of BMI, isolating the waist-circumference signal that BMI alone misses.

What this calculator does

BMI uses only height and weight, so two people with the same BMI can have very different body shapes and very different health risk profiles depending on where their weight is distributed. This tool calculates four separate research-published metrics that try to capture that missing information — ABSI, BRI, RFM and BAI — as toggle options in one calculator, since each uses different inputs and has its own formula and provenance.

The four formulas

A Body Shape Index (ABSI), published by Krakauer & Krakauer in 2012, combines waist circumference, BMI and height specifically so that the result is statistically independent of BMI — isolating the waist-circumference signal BMI alone can't see. Body Roundness Index (BRI), from Thomas et al., 2013, models the body as an ellipse using waist circumference and height to estimate overall "roundness." Relative Fat Mass (RFM), from Woolcott & Bergman's 2018 study, is a simple formula using only height, waist and sex, validated against DEXA scans as a direct %body-fat estimator. Body Adiposity Index (BAI), from Bergman et al., 2011, uses only hip circumference and height — designed as a scale-free alternative to BMI that doesn't require weighing yourself at all.

Why they're grouped in one tool

Each formula answers a version of the same underlying question — "is BMI missing something about my body shape or fat distribution?" — using a different combination of inputs. Rather than publishing four near-identical calculator pages, this hub groups them as a formula toggle, the same pattern used for BMR and ideal-weight formulas elsewhere.

Reading your result

RFM and BAI were both explicitly designed to approximate %body fat, so this tool shows their result alongside the same category bands (essential fat, athletic, fitness, average, above average) used by the body-fat-calculator elsewhere in this hub. ABSI and BRI are continuous shape indices without an official simple category scale — showing an invented category for them would overstate the precision of the underlying research, so this tool reports their raw value with a plain-language interpretation instead, useful mainly for tracking your own trend over time.

How these compare to BMI

Research on all four generally finds they correlate at least as well as BMI with metabolic risk markers in the populations studied, because they incorporate information about fat distribution — central versus peripheral — that a height-and-weight-only formula structurally can't capture. None of them is a diagnostic test, and none has the decades of population-level validation data that BMI has accumulated; they're newer research tools that are still being studied.

Not medical advice

These are general body-composition screening estimates from published research formulas, not medical or diagnostic measurements. Use them to understand your own body shape trends over time, and discuss any health concerns with a qualified healthcare provider.

Frequently asked questions

Why offer four different formulas in one tool instead of separate calculators?: Each is a distinct published research metric with its own formula and required inputs, but they answer the same underlying question — 'is BMI missing something about my body shape?' — so they're grouped as toggle options in one tool rather than four near-identical pages, the same pattern this hub uses for BMR and ideal-weight formulas.
Which one should I use?: RFM (Relative Fat Mass) needs the fewest inputs (height, waist, sex) and was validated against DEXA scans as a %body-fat estimator, making it a reasonable default. ABSI and BRI are shape indices rather than %fat estimators — useful for tracking your own trend over time rather than comparing to a fixed number.
Are these more accurate than BMI?: Research on all four generally finds they correlate at least as well as BMI with metabolic risk markers in the populations studied, because they incorporate fat distribution (central vs. peripheral) that BMI's height-and-weight-only formula can't see. None of them is a diagnostic test.
Why don't ABSI and BRI show a category label like RFM and BAI do?: ABSI and BRI were published as continuous shape indices without an official simple category scale (unlike RFM and BAI, which were explicitly designed to approximate %body fat). Showing an invented category for them would overstate the precision of the underlying research, so this tool reports the raw value with a plain-language interpretation instead.

These calculators give general estimates from published formulas. They are not medical advice and do not account for your individual health; talk to a qualified professional before changing your diet or training.