Last updated: 2026-07-08

Relative Fat Mass

Relative Fat Mass (RFM) estimates whole-body fat percentage from just two measurements: height and waist circumference. It was introduced by Orison Woolcott and Richard Bergman in 2018, who searched a large set of anthropometric ratios for the one that best matched body fat measured by DXA scanning and arrived at a single, simple equation. Because it relies on the waist rather than on weight, it reflects central fat and requires no scale and no skinfold calipers.

What the estimate represents

RFM reports an approximate body-fat percentage — the share of total body mass made up of fat. The key driver is the ratio of height to waist: a wider waist relative to height lowers that ratio and raises the estimated fat percentage. A separate constant is added for women, reflecting that women carry more essential and total fat than men at the same height-to-waist ratio.

The formula

Here $h$ is height and $\mathrm{WC}$ is waist circumference, both in the same unit so the ratio is dimensionless. The sex term $S$ is zero for men and one for women, so the equation adds twelve percentage points for women. The constants were fitted against DXA reference values rather than derived from geometry, which is why the result is read directly as a fat percentage.

Worked example

Consider a man with a height of 1.75 m5 ft 9 in and a waist of 0.90 m35 in. Working in metres:

An estimate near twenty-five percent sits toward the upper part of the typical range for an adult man.

Interpreting the number

Healthy body-fat ranges are sex- and age-specific, and there are no fixed bands built into this estimate. As a broad reference, a healthy range for men is often placed at roughly fourteen to twenty-four percent and for women at roughly twenty-one to thirty-one percent, with athletes frequently lower and the ranges drifting upward with age. The sex-specific gap exists because a portion of female fat is essential reproductive and structural tissue. For one person, the trend across several measurements usually says more than a single reading compared against population averages.

Limits

RFM is a statistical estimate, not a body-composition measurement. It was fitted on a specific study population and is less reliable at the extremes of body size, and it cannot separate subcutaneous from visceral fat. It is best read alongside other measures such as the Body Fat % Calculator (Navy Method), the Waist-to-Height Ratio Calculator, and the Body Adiposity Index Calculator, and any health decision should rest on a professional assessment rather than a single figure.