Relative Fat Mass (RFM) Calculator

Background.

Relative fat mass, or RFM, estimates what percentage of your body is fat using two measurements and your sex: your height, and your waist circumference measured at the top of your hip bone. There is no weight, no neck measurement, no skinfold calliper and no scale involved. Enter the two numbers and the calculator returns an estimated whole-body fat percentage, where that sits against the published obesity cut-offs, and — deliberately in the open — the waist-to-height ratio the whole equation is built on. Read the number as a screening estimate, not a measurement and not a diagnosis: it was fitted against DXA scans in American adults aged 20 and over, it has not been validated in pregnancy, and there is a separate published equation for children and adolescents that this page does not implement.

RFM was published by Orison Woolcott and Richard Bergman in Scientific Reports in 2018. They developed it on 12,581 adults from the US National Health and Nutrition Examination Survey collected between 1999 and 2004, then validated it on a separate 3,456 adults from the 2005–2006 cycle, in both cases against whole-body fat measured by dual-energy X-ray absorptiometry. The equation that came out is startlingly simple: RFM = 64 − 20 × (height ÷ waist) + 12 × sex, where sex is 0 for men and 1 for women and height and waist are expressed in the same units. Against DXA in the validation set it achieved an R² of 0.75 in men and 0.69 in women, with a bias of half a percentage point in men and nine-tenths in women, and it placed 88.9 % of men's and 91.5 % of women's estimates within 20 % of the DXA value. In the same comparison it outperformed body mass index and several published anthropometric equations.

It is worth being straightforward about what that simplicity implies. Because the only variables are height, waist and sex, RFM is a fixed transformation of your waist-to-height ratio plus a 12-point offset for women. This calculator prints that ratio next to the result rather than hiding it. What RFM adds over the ratio alone is calibration: the numbers 64, 20 and 12 are a regression against actual DXA-measured fat mass, so RFM converts a shape ratio into a quantity you can compare with a body-fat percentage from a scan or a calliper. What it cannot add is information the ratio does not contain. Two people with the same sex and the same waist-to-height ratio get the same RFM no matter how much they weigh or how muscular they are, and that is a genuine limitation, not a rounding detail.

An independent group tested this. Guzmán-León and colleagues published an external validation in PLOS ONE in 2019 using 61 adults in north-west Mexico and four different reference methods. Against DXA their finding backed the original: the regression of RFM on DXA gave y = 1.12 + 0.99x with R² = 0.84, and the intercept of 1.12 was not significantly different from zero. Against air-displacement plethysmography, bioelectrical impedance and a four-compartment model, however, the intercepts were −9.95, −12.6 and −13.6 and all differed significantly from zero. The honest reading is that RFM agrees with DXA, because DXA is what it was fitted to; it does not automatically agree with every other way of measuring body fat. If you are comparing an RFM result with a number from a Bod Pod or an impedance scale, expect a gap of several percentage points that is a property of the methods, not of your body.

Where you put the tape is the single largest source of error on this page, and it differs from the instruction on our body fat calculator on purpose. RFM's coefficients were fitted on NHANES waist measurements, and the NHANES Anthropometry Procedures Manual specifies the uppermost lateral border of the right hip bone — the iliac crest — with the tape horizontal at the midaxillary line and the reading taken at the end of a normal breath out. That is a different place from the World Health Organization's recommended midpoint between the lowest rib and the iliac crest, and a different place again from the navel, which is where the US Navy circumference method has men measure. Feeding the equation a waist taken somewhere else is feeding it a number it was not calibrated for. Measure at the hip bone, keep the tape level, and do not hold your breath in.

In a follow-up paper in the International Journal of Obesity in 2020, the same authors looked at what RFM value predicts death. Using 31,008 NHANES adults followed for a median of 8.3 years, over which 2,517 died, they identified optimal cut-offs of 40.8 % and 41.6 % for women and 30.9 % and 28.9 % for men depending on the statistical criterion, and recommended rounding these to 40 % for women and 30 % for men to diagnose obesity. This calculator uses those rounded values and tells you which one it applied and by how much you are above or below it. It does not tell you that you are obese, healthy or at risk, because a cut-off derived from population mortality statistics describes groups, not the person reading the screen. If your result sits near or above the cut-off, that is a reason to bring it to a clinician alongside your blood pressure, your fasting glucose and your lipid panel — not a conclusion on its own.

What is relative fat mass (rfm) calculator?

Relative fat mass (RFM) is an anthropometric estimator of whole-body fat percentage calculated as 64 − 20 × (height ÷ waist circumference) + 12 × sex, where sex takes the value 0 for men and 1 for women and both measurements are expressed in the same unit. It was introduced by Woolcott and Bergman in Scientific Reports in 2018 as a replacement for body mass index in estimating adiposity. The authors screened 365 candidate anthropometric indices against dual-energy X-ray absorptiometry in 12,581 adults from NHANES 1999–2004, selected the height-to-waist form on performance, and validated it in an independent 3,456 adults from NHANES 2005–2006. RFM's appeal is that it requires no scale, no callipers and no electrical device — a single tape measure and a known height are enough — while estimating the same quantity a DXA scan reports. Because it substitutes waist circumference for body weight, it is sensitive to central adiposity in a way BMI structurally is not: a person who has replaced fat with muscle at constant weight will show a falling RFM and an unchanged BMI. Its principal limitation is the flip side of its simplicity. RFM is a fixed function of the waist-to-height ratio and sex, so it carries exactly the information those two variables carry and no more; it cannot distinguish two people of identical shape and different weight, and it has no way of registering peripheral fat on the hips and thighs. A separate paediatric equation was published by the same authors in 2019, which is why the adult equation on this page should not be applied to anyone under 20.

How to use this calculator.

1. 01Select your sex. The equation adds a flat 12 percentage points for women — that is the sex term from the published regression, not an adjustment we invented.
2. 02Measure your waist at the top edge of your hip bone (the iliac crest), on the side of your body, with the tape horizontal all the way round. This is the NHANES measurement site the equation was fitted on.
3. 03Take the reading at the end of a normal breath out, with the tape snug but not compressing the skin. Do not suck in and do not measure over bulky clothing.
4. 04Enter your standing height in the same unit you used for the waist, then confirm the unit selector matches. RFM depends only on the ratio, so centimetres and inches give an identical answer — but mixing them gives nonsense, which is why the calculator rejects impossible ratios.
5. 05Read the RFM percentage as an estimate of whole-body fat calibrated against DXA scans.
6. 06Check the cut-off tiles: the calculator names the published cut-off for your sex (30 % men, 40 % women) and how far above or below it you are.
7. 07Look at the waist-to-height ratio tile to see the raw signal the estimate is built from. If it reads near or above 0.5, that is worth knowing on its own.
8. 08Repeat with the same tape, at the same landmark, at the same time of day when you want to track change. Site drift between measurements will swamp any real change.

The formula.

The equation is RFM = 64 − 20 × (height ÷ waist) + 12 × sex, with sex = 0 for men and sex = 1 for women, and with height and waist in the same unit. Because it depends only on the ratio, no unit conversion is performed or needed: 90 cm on 178 cm and 35.43 in on 70.08 in give exactly the same answer. Substituting for men gives RFM = 64 − 20 ÷ WHtR and for women RFM = 76 − 20 ÷ WHtR, where WHtR is the ordinary waist-to-height ratio — the calculator prints both forms' inputs so you can check this yourself. Two directional facts, both verified against the equation rather than asserted: a larger waist at fixed height lowers height ÷ waist and therefore RAISES RFM, and a taller person with the same waist raises height ÷ waist and therefore LOWERS RFM. Working the shipped example: a man with a 90 cm waist and 178 cm height has height ÷ waist = 1.9777777778, so 20 × that is 39.5555555556, and RFM = 64 − 39.5555555556 = 24.4444444444 %. His waist-to-height ratio is 0.5056179775. Against the men's cut-off of 30 % he is 5.5555555556 points below. ROUNDING STAGE: every step is carried at full decimal precision and rounded once, at the return — no intermediate rounding. The obesity classification is decided on the unrounded value and the comparison is inclusive, so exactly 30.000000 % classifies as at the cut-off while 29.999999 % does not. DOMAIN LIMIT: the equation is linear in the reciprocal of the waist-to-height ratio, so it returns negative percentages at extreme ratios — below about 0.3125 for men. The calculator therefore rejects a waist outside 34 % to 110 % of height and says why, rather than printing an impossible number.

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 consulted for this build: 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.