Estimate your body fat percentage from just your height and waist circumference using the Relative Fat Mass (RFM) formula — a two-measurement alternative to BMI.
Relative Fat Mass (RFM) is a formula that estimates your whole-body fat percentage using only your height and waist circumference — no scale, no skinfold calipers, and no scan required. It was introduced by researchers Diana Thomas, along with Orison Woolcott and Richard Bergman, in a 2018 study published in Scientific Reports, as a simple alternative to BMI that correlates more closely with body fat measured by DEXA scanning in the study's validation sample. RFM uses separate formulas for men and women to reflect typical differences in fat distribution between the sexes, and produces a percentage figure you can compare against reference cutoffs used in later research.
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RFM uses your height divided by your waist circumference (both in the same unit), multiplies that ratio by 20, and subtracts the result from a sex-specific starting constant. The starting constant is higher for women than for men, which is what produces the sex difference in the final percentage.
Men and women tend to store body fat differently even at similar body sizes — women naturally carry a higher proportion of essential and subcutaneous fat, particularly around the hips and thighs, while men tend to store proportionally more fat centrally around the abdomen at a given waist size. Because RFM is built from only two inputs, it accounts for this by starting from a different baseline constant for each sex (64 for men, 76 for women) rather than by adding a separate variable, so the same height-to-waist ratio maps to a higher estimated fat percentage for a woman than for a man.
Traditional body fat estimates typically need your weight, and often several skinfold sites or a piece of equipment such as bioelectrical impedance scales. RFM was deliberately built around only two measurements — height and waist circumference — because waist circumference already reflects central body fat fairly directly, and expressing it relative to height removes most of the distortion that comes from simply comparing people of different statures. Leaving weight out of the formula also means RFM is not affected by how much of a person's body mass is muscle rather than fat, which is one of the main criticisms of BMI.
A separate follow-up study by Woolcott, published in 2020 in the International Journal of Obesity, examined how RFM related to mortality risk in a large population sample and reported cutoff values commonly cited as obesity screening reference points: an RFM of 30% or higher for men, and 40% or higher for women. These figures are population-level research reference points, not individual diagnostic thresholds — sitting below the cutoff does not automatically mean a person's body composition is "healthy," it simply means their estimated RFM falls under that particular research benchmark.
| Sex | Commonly Cited Cutoff | Source Context |
|---|---|---|
| Men | RFM ≥ 30% | Obesity screening reference point, Woolcott 2020, International Journal of Obesity |
| Women | RFM ≥ 40% | Obesity screening reference point, Woolcott 2020, International Journal of Obesity |
RFM is designed to approximate the proportion of your total body weight that is fat, expressed as a percentage, without directly weighing you or measuring your body fat with equipment. It does this indirectly, by using the relationship between your waist circumference and your height as a proxy for central body fat relative to frame size. In the original 2018 study, RFM values were compared against body composition results from dual-energy X-ray absorptiometry (DEXA) scans, and the formula was found to track DEXA-measured fat percentage more closely across the study sample than BMI did, which is the main reason it has drawn interest as a simple alternative screening figure.
| Measure | Inputs | Output |
|---|---|---|
| RFM | Height, waist circumference, sex | Estimated body fat percentage |
| BMI | Height, weight | A weight-to-height index, not a fat percentage |
BMI cannot distinguish fat from muscle because it never measures either one directly — it only relates total weight to height. RFM does not use weight at all, and instead leans on waist circumference as a proxy for central fat, combined with a sex-specific adjustment. This means RFM gives you a figure that is intended to represent body fat percentage directly, whereas a BMI figure is really only a screening index that happens to correlate with body fat at a population level.
Because RFM leaves weight out of the calculation entirely, it tends to hold up better than BMI for muscular individuals in most everyday cases — a heavily muscled person with a lean waist will generally get a plausible RFM reading rather than being pushed into an "overweight" BMI category simply because of muscle mass. That said, RFM is still an equation built on population averages, and it can still be skewed for people with unusual body proportions, very low or very high waist-to-height ratios, or highly trained athletes whose central fat distribution does not match the general population the formula was built on.
What is Relative Fat Mass (RFM)?
Relative Fat Mass (RFM) is a formula that estimates your whole-body fat percentage using only your height, waist circumference, and sex. It was introduced by Woolcott and Bergman in a 2018 Scientific Reports paper as a simple, equipment-free alternative to BMI for approximating body fat percentage.
How accurate is RFM compared with BMI?
In the original 2018 validation study, RFM correlated more closely with body fat percentage measured by DEXA scanning than BMI did, particularly because it does not confuse muscle mass with fat mass. It is still an estimate rather than a direct measurement, so its accuracy for any one individual will vary, and it should be treated as a useful screening figure rather than a lab-grade result.
Why does the RFM formula differ for men and women?
Men and women tend to distribute body fat differently even at similar waist-to-height ratios, with women naturally carrying a higher proportion of body fat overall. RFM accounts for this with a different starting constant in the formula — 64 for men and 76 for women — so the same height-to-waist ratio produces a higher estimated fat percentage for a woman than for a man.
How do I measure my waist circumference correctly for this calculator?
Measure at the narrowest point of your torso, which for most people is roughly level with the belly button, midway between the bottom of the ribs and the top of the hip bones. Breathe out normally before reading the tape, keep it horizontal and snug against the skin without pulling it tight, and take two or three readings to average for a more reliable result.
Is RFM the same as body fat percentage from a DEXA scan?
No. RFM is an estimate calculated from height and waist circumference, while a DEXA scan directly measures bone, lean tissue, and fat mass using low-dose X-ray imaging. RFM was designed to approximate what a DEXA scan would show, and research suggests it tracks DEXA results more closely than BMI does, but a scan remains the more accurate measurement for any individual.
Can athletes or very muscular people get skewed RFM results?
RFM tends to be more forgiving than BMI for muscular individuals because it does not use body weight at all. However, it is still based on population-level averages, so athletes with unusual body proportions or central fat distribution that differs from the general population used to build the formula may still see results that don't fully reflect their actual body composition.
What's a healthy RFM range?
RFM research has not defined a single universal "healthy range" in the way some other measures do. The most commonly cited reference points are obesity screening cutoffs from a 2020 Woolcott mortality study — around 30% for men and 40% for women. Falling below these cutoffs does not automatically mean a person's body composition is optimal; it only means their estimated RFM sits under that specific research benchmark.
What are the main limitations of RFM?
RFM is an equation derived from population data, so it will not be perfectly accurate for every individual, particularly people with body proportions that differ from those in the original validation studies, such as very tall or very short individuals, pregnant women, or highly trained athletes. It also depends heavily on an accurate waist measurement — a poorly measured waist will directly skew the result. It should be used as a screening estimate alongside other health information, not as a stand-alone diagnosis.