Relative Fat Mass:
Table of Contents
Relative Fat Mass Chart
Calculate body fat percentage using the relative fat mass index calculator and compare your results against the rfm chart below. The values vary by gender so make sure to check your result against the appropriate column.
| Gender | Male | Female |
|---|---|---|
| Athletic | 6-13% | 14-20% |
| Healthy | 14-17% | 21-24% |
| Normal | 18-24% | 25-31% |
| Overweight | > 25% | > 32% |
A newer, more accurate body fat estimate
RFM (Relative Fat Mass) was introduced in 2018 by researchers at Cedars-Sinai Medical Center who validated it against thousands of DEXA scans — the gold standard for body fat measurement. It outperformed BMI as a predictor of actual body fat percentage, using only height and waist measurements. It's the most accurate "no scale needed" body fat estimate available.
How RFM is calculated
The formula differs by sex because typical body composition differs:
Men: RFM = 64 − 20 × (Height ÷ Waist)
Women: RFM = 76 − 20 × (Height ÷ Waist)
Height and waist in the same unit (cm or inches; the ratio is unitless). The result is an estimated body fat percentage.
Worked example using the calculator's defaults (180 cm height, 86 cm waist, male):
- Height ÷ Waist = 180 ÷ 86 = 2.093
- RFM = 64 − 20 × 2.093 = 64 − 41.86 = 22.1%
That lands at the boundary between "Normal" and "Overweight" on the RFM chart for adult men.
Why RFM was developed
The standard body fat estimation formulas (Deurenberg, US Navy circumference method, skinfolds) all had known accuracy problems — particularly for older adults and people outside the populations the formulas were originally developed on.
In 2018, Woolcott and Bergman at Cedars-Sinai published a paper in Scientific Reports using NHANES data (a large US health survey including DEXA body composition scans) to look for the simplest possible accurate formula. They tested 365 anthropometric measurements against DEXA-measured body fat and found that height and waist circumference alone, combined with sex, produced the lowest error of any practical formula.
Their RFM formula:
- Reduced prediction error compared to BMI for both men and women.
- Required only two measurements (no scale, no calipers).
- Validated specifically against DEXA, the most accurate non-invasive body fat method.
The result was a much-simpler, more-accurate body fat estimate. It's gradually being adopted in clinical research and is increasingly available in consumer fitness tools.
How to measure waist correctly
RFM's accuracy depends almost entirely on a correct waist measurement. The standard reference point: the midpoint between the bottom of your lowest rib and the top of your hip bone (iliac crest) — usually just above the navel, not at it. Practical steps:
- Stand relaxed, feet shoulder-width apart, no clothing in the way.
- Breathe out normally; don't hold your stomach in.
- Wrap a flexible tape horizontally around your waist at the reference point.
- Snug but not compressing skin.
- Take three measurements and use the average.
For tracking changes over time, measure under the same conditions each time — ideally morning, after using the bathroom, before eating, in a consistent state of hydration.
RFM vs other body fat estimates
| Method | Inputs needed | Accuracy vs DEXA |
|---|---|---|
| BMI-based estimate | Height, weight, age, sex | ±5–7% |
| RFM (this calculator) | Height, waist, sex | ±3–5% |
| US Navy circumference | Height, waist, neck (+hip for women), sex | ±3–4% |
| Skinfold (3-site Jackson-Pollock) | 3 skinfold measurements, age, sex | ±3–5% (tester-dependent) |
| Bioimpedance (consumer scales) | Weight, height, age, sex (electrical reading) | ±3–8% (hydration-dependent) |
| DEXA scan | Lab equipment | Reference standard |
RFM is competitive with US Navy and skinfold methods while requiring fewer inputs and no technique-dependent measurements. For most people, it's the practical sweet spot between accuracy and convenience.
Where RFM has limits
Like any formula-based estimate, RFM works best for the population it was developed on. Cases where it's less reliable:
- Athletes with high muscle mass. The Woolcott validation included general adults, not elite athletes. Highly muscular individuals may have RFM readings that overestimate their actual body fat.
- Very lean individuals (under 8% body fat for men, 14% for women). RFM formulas can give negative or implausibly low readings at the extremes.
- Pregnant women. Waist measurement isn't applicable during pregnancy.
- Pediatric populations. The formula was validated on adults; children need different formulas based on percentile growth charts.
- People with significant abdominal asymmetry (post-surgery, certain medical conditions). Waist measurement assumes typical abdominal anatomy.
For these groups, DEXA or skinfold methods are typically more reliable.
Sources & references
- Woolcott OO, Bergman RN (2018). "Relative fat mass (RFM) as a new estimator of whole-body fat percentage — a cross-sectional study in American adult individuals." Scientific Reports 8:10980 — the original RFM validation paper.
- NIH NCBI — full text of Woolcott & Bergman 2018
- CDC NHANES 1999–2004 — underlying body composition data used in RFM validation.
FAQs
For estimating body fat percentage specifically, yes — that's what the Woolcott & Bergman 2018 study established. They compared RFM against DEXA scans in a large sample and found RFM had lower error than BMI when predicting actual body fat percentage. But RFM and BMI measure different things: BMI predicts overall mortality risk fairly well; RFM specifically predicts body fat. Neither replaces the other — they complement each other.
The Woolcott team ran a large statistical analysis against DEXA-measured body fat and found those two measurements alone (plus sex) predicted body fat percentage better than more complex formulas using weight, BMI, age, or skinfold measurements. The waist captures abdominal fat distribution; the height-to-waist ratio captures fat relative to body size. Together they encode most of what other formulas need extra inputs for.
Waist-to-height ratio (WHtR) is just waist ÷ height, giving a number like 0.48. RFM uses the same inputs but transforms them into an estimated body fat percentage (a number like 22%), with different coefficients for men and women. WHtR is useful for risk screening (under 0.5 is healthy); RFM is useful for body composition estimation. RFM is harder to compute mentally but produces a more interpretable number (a body fat percentage you can compare to known healthy ranges).
Like all formula-based body fat estimates, it doesn't directly measure body fat — it predicts it from proxy measurements. For people whose body composition is unusual (athletes with very high muscle mass, very lean individuals, certain medical conditions affecting fluid retention), the prediction can be off. The Woolcott validation was done on a general adult population, not athletes. For athletic populations, DEXA remains the gold standard.
Yes, and it's actually well-suited for tracking. Because RFM only needs waist and height (height doesn't change in adults), all the variation comes from changes in your waist circumference — which is the body composition signal you care about. Watching RFM change over months gives a clearer picture of body fat change than the noisier scale weight. Take the waist measurement under consistent conditions (morning, before eating, breathed out normally) for the most reliable trend.