Relative Fat Mass (RFM) Calculator

Background.

Body mass index is a weak guide to how much fat a woman carries. When Orison Woolcott and Richard Bergman checked it against DXA body scans in a US national survey sample, BMI put 56.5% of women on the wrong side of a body-fat definition of obesity. Their alternative, relative fat mass, misclassified 12.7%, and it needs nothing more than a tape measure and your height.

The equation, published in Scientific Reports in 2018, is RFM = 64 − 20 × (height ÷ waist), with 12 added for women. The authors tested 365 candidate formulas built from body measurements on 12,581 adults from the National Health and Nutrition Examination Survey (NHANES) of 1999–2004, kept the one that best matched DXA, and confirmed it on a separate 3,456 adults from the 2005–2006 survey. The answer is an estimate of the fat percentage a DXA scanner would report, not a reading of your own tissue.

Alongside the estimate you see the obesity cut-off for your sex (30% for men, 40% for women, from the same authors' 2020 mortality study), how far above or below it you sit, and the two ratios the equation is made of. Those ratios are shown because they are all RFM uses. With the sex term fixed, RFM is the waist-to-height ratio placed on a body-fat scale: a waist exactly half your height gives 24% for a man and 36% for a woman, whatever either of them weighs.

The equation was built for adults aged 20 and over, and the survey groups it was fitted on were Mexican-American, White and African-American. A separate equation exists for children and adolescents, and none applies in pregnancy. Read the result as a screening estimate rather than a diagnosis: a figure near or over the cut-off is worth taking to a clinician together with blood pressure, blood sugar and cholesterol results, which say far more about health risk than any single body-fat number.

What is relative fat mass (rfm) calculator?

Relative fat mass is an estimate of body fat percentage: the share of total body weight that is fat, expressed in percent, on the scale used by dual-energy X-ray absorptiometry (DXA). DXA passes two low-dose X-ray beams of different energies through the body and separates fat, lean tissue and bone by how much of each beam they absorb, which is why it serves as the reference in most body composition research. RFM tries to reproduce that number from a waist measurement, a height and sex alone.

It measures something different from BMI. BMI divides weight by height squared and cannot tell muscle from fat; RFM never asks your weight, and leans instead on the waist, where much of the body's excess fat is stored. The price of that simplicity is that RFM sees only the trunk. Fat carried mainly on the hips and thighs, more typical of women, and genuine differences in build between people of the same shape are invisible to it, so two people of the same sex with identical waist-to-height ratios always receive the same estimate.

How to use this calculator.

1. 01Choose your sex. The published equation adds a fixed 12 percentage points for women; there is no other adjustment.
2. 02Measure your waist at the top of the hip bone, the iliac crest: feel down your side until you meet the bony ridge, run the tape horizontally around the body at that level, keep it snug on bare skin without pressing in, and read it at the end of a normal breath out. This is the NHANES site the coefficients were fitted on, which is lower than the WHO midpoint below the ribs and different from the navel.
3. 03Enter your standing height, without shoes, in the same unit as the waist. The answer depends only on the ratio, so centimetres and inches give the same result, but both numbers must use the unit you select (40–200 cm or 15.7–78.8 in for the waist, 120–230 cm or 47.2–90.6 in for height).
4. 04Read the RFM percentage, then the cut-off line beneath it, which names the threshold applied to your sex and how many percentage points above or below it you are.
5. 05Look at the waist-to-height ratio as well. It is the raw signal behind the estimate, and a ratio of 0.5 or more is widely used on its own as a simple sign of raised health risk.
6. 06To follow change over months, measure at the same landmark, at the same time of day, with the same tape. At typical proportions about 2.3 cm of waist moves RFM by one point, so a tape placed a few centimetres higher or lower can hide or invent progress.

The formula.

The equation is RFM = 64 − 20 × (height ÷ waist) + 12 × sex, with sex = 0 for men and 1 for women and both lengths in the same unit. Written separately, men get RFM = 64 − 20 ÷ WHtR and women RFM = 76 − 20 ÷ WHtR, where WHtR is waist divided by height. The constants 64, 20 and 12 are regression coefficients from Woolcott and Bergman's 2018 analysis, chosen to make the estimate agree with DXA-measured fat percentage in the NHANES development sample.

Because height sits on top of the fraction, a bigger waist lowers height ÷ waist and raises RFM, while greater height at the same waist lowers it. The effect is not a straight line: the change per centimetre of waist is 20 × height ÷ waist², so it is largest for slim waists. For a 178 cm man with a 90 cm waist, each extra centimetre of waist adds about 0.44 points and each extra centimetre of height subtracts about 0.22. Errors in the tape matter roughly twice as much as errors in height.

The linear form also explains the range of ratios accepted. As the waist shrinks relative to height, the equation keeps falling and would reach 0% for a man at a WHtR of 0.3125, an impossible result. Waists outside 34% to 110% of height are therefore refused, which keeps male estimates between about 5.2% and 45.8% and female estimates between about 17.2% and 57.8%, comfortably covering the adult range the equation was fitted on.

In the 2005–2006 validation sample, RFM explained 75% of the variation in DXA fat percentage among men and 69% among women (BMI managed 61% and 65%), with an average bias of 0.5 and 0.9 points, and 88.9% of men's and 91.5% of women's estimates fell within 20% of the DXA value. Those are group statistics: an individual can still be several points away from a scan.

The cut-off comparison is inclusive, so exactly 30% for a man or 40% for a woman counts as at the cut-off.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Woolcott OO, Bergman RN. "Relative fat mass (RFM) as a new estimator of whole-body fat percentage — A cross-sectional study in American adult individuals." Sci Rep. 2018;8:10980. PMID 30030479, PMC6054651. Source of the equation RFM = 64 − 20 × (height/waist) + 12 × sex, the sex coding, the NHANES 1999–2004 development sample (n = 12,581) and 2005–2006 validation sample (n = 3,456), and the DXA accuracy statistics quoted on this page. Open access.
2. [2]Woolcott OO, Bergman RN. "Defining cutoffs to diagnose obesity using the relative fat mass (RFM): Association with mortality in NHANES 1999–2014." Int J Obes (Lond). 2020;44(6):1301–1310. PMID 31911664. Source of the rounded obesity cut-offs (40 % women, 30 % men) and the four unrounded optima (40.8 %, 41.6 %, 30.9 %, 28.9 %), from 31,008 adults with 2,517 deaths over a median 8.3-year follow-up. Abstract open access; full text paywalled.
3. [3]Guzmán-León AE, Velarde AG, Vidal-Salas M, Urquijo-Ruiz LG, Caraveo-Gutiérrez LA, Valencia ME. "External validation of the relative fat mass (RFM) index in adults from north-west Mexico using different reference methods." PLOS ONE. 2019;14(12):e0226767. Independent second authority: reproduces the equation and sex coding exactly, confirms RFM beats BMI against all four reference methods, and reports the DXA-specific agreement (intercept 1.12, 95 % CI −2.44 to 4.68) versus significant bias against ADP, BIA and the 4-compartment model. Open access.
4. [4]Woolcott OO, Bergman RN. "Relative fat mass as an estimator of whole-body fat percentage among children and adolescents: A cross-sectional study using NHANES." Sci Rep. 2019;9:15279. PMID 31649287. Establishes that a separate paediatric equation exists, which is why the adult equation on this page is restricted to ages 20 and over. Abstract open access.
5. [5]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 measurement instruction used on this page: the uppermost lateral border of the right ilium at the midaxillary line, tape in a horizontal plane, read at the end of normal expiration.
6. [6]World Health Organization — "Waist Circumference and Waist–Hip Ratio: Report of a WHO Expert Consultation," Geneva, 8–11 December 2008 (published 2011). Cited for the competing measurement protocol (midpoint between the lowest rib and the iliac crest), which this page deliberately does not use because the RFM coefficients were fitted on NHANES iliac-crest measurements.