What is Relative Fat Mass?
Relative Fat Mass (RFM) is a body fat estimation formula developed by Dr. Orison Woolcott and Dr. Richard Bergman at Cedars-Sinai Medical Center, published in Scientific Reports in 2018. It estimates whole-body fat percentage using only two easily measured anthropometric variables: height and waist circumference.
RFM was developed from a cohort of over 12,000 adults in the NHANES (National Health and Nutrition Examination Survey) database and validated against DEXA (dual-energy X-ray absorptiometry) scans, the gold standard for body composition measurement. The study found that RFM predicted body fat percentage more accurately than BMI, especially when validated against DEXA measurements.
RFM Formula (Woolcott-Bergman Equation)
The RFM formula differs by sex:
Both height and waist circumference must be in the same unit (cm or meters). The result is expressed as a percentage of body fat. The constant difference of 12 between the male (64) and female (76) formulas reflects the fact that women naturally carry more body fat than men at any given body proportion.
Body Fat Categories
Men
| Category | Body Fat % | Description |
|---|---|---|
| Essential Fat | 2 – 5% | Minimum fat needed for basic physiological function |
| Athletes | 6 – 13% | Competitive athletes; lean and muscular |
| Fitness | 14 – 17% | Physically fit; visible muscle definition |
| Average | 18 – 24% | Typical healthy adult range |
| Obese | 25%+ | Excess body fat; increased health risks |
Women
| Category | Body Fat % | Description |
|---|---|---|
| Essential Fat | 10 – 13% | Minimum fat needed for hormonal and reproductive function |
| Athletes | 14 – 20% | Competitive athletes; lean physique |
| Fitness | 21 – 24% | Physically fit; healthy body composition |
| Average | 25 – 31% | Typical healthy adult range |
| Obese | 32%+ | Excess body fat; increased health risks |
RFM vs BMI Comparison Diagram
RFM vs BMI: Key Differences
| Feature | RFM | BMI |
|---|---|---|
| Inputs | Height + waist circumference | Height + weight |
| Output | Body fat percentage estimate | Weight-to-height ratio (kg/m²) |
| Sex-specific | Yes (different formulas) | No |
| Waist measurement | Required | Not used |
| Affected by muscle mass | Less affected | Significantly affected |
| Central obesity detection | Yes (waist-based) | No |
| Correlation with DEXA | r = 0.86 (men), 0.85 (women) | r = 0.73 (men), 0.75 (women) |
| Equipment needed | Tape measure + height | Scale + height |
Body Composition Methods
Several methods exist for measuring body composition, ranging from simple anthropometric measures to advanced imaging:
| Method | Accuracy | Cost | Accessibility |
|---|---|---|---|
| DEXA scan | Very high (gold standard) | High ($100-300) | Clinical setting |
| Hydrostatic weighing | High | Moderate | Research labs |
| Air displacement (Bod Pod) | High | Moderate | Specialized facilities |
| Bioelectrical impedance (BIA) | Moderate | Low-Moderate | Gyms, clinics, home scales |
| Skinfold calipers | Moderate (operator-dependent) | Low | Anywhere |
| RFM | Moderate (validated vs DEXA) | Free | Anywhere (tape measure only) |
| BMI | Low (indirect) | Free | Anywhere (scale only) |
How to Measure Waist Circumference
Accurate waist circumference measurement is critical for RFM accuracy. Follow these steps:
- Stand upright with feet shoulder-width apart and arms at your sides
- Remove or lift clothing from the waist area
- Locate the top of your hip bone (iliac crest) on both sides
- Place the measuring tape horizontally around your waist at the level of the iliac crest (approximately at the navel or just above)
- Keep the tape snug but not compressing the skin
- Breathe normally and take the measurement at the end of a normal exhalation
- Record the measurement to the nearest 0.1 cm or 0.5 inch
Worked Example
A 35-year-old male with height 175 cm and waist circumference 88 cm:
= 64 − (20 × 1.989)
= 64 − 39.77
= 24.2% body fat
This falls within the Average category for men (18-24%). If this same person weighed 80 kg, their BMI would be 80 / 1.75² = 26.1 (overweight by BMI). This illustrates how RFM and BMI can give different pictures — RFM suggests average body fat while BMI flags the person as overweight, potentially due to higher muscle mass.
Frequently Asked Questions
Is RFM better than BMI?
For estimating body fat percentage, yes. Studies show RFM has a stronger correlation with DEXA-measured body fat (r ~0.85) compared to BMI (r ~0.74). RFM is also sex-specific and incorporates waist circumference, which captures central adiposity. However, BMI remains useful as a population-level screening tool due to its simplicity and extensive validation.
Does RFM work for athletes?
RFM performs better than BMI for muscular individuals because it uses waist circumference rather than weight. An athlete with high muscle mass but a lean waist will get a more realistic body fat estimate from RFM than BMI. However, for very lean or very muscular athletes, DEXA or hydrostatic weighing will provide more precise measurements.
What if my RFM seems too high or too low?
Common causes of inaccurate RFM results include: incorrect waist measurement technique, measuring over clothing, measuring at the wrong anatomical landmark, or postprandial bloating. Re-measure your waist following the proper technique described above. RFM may also be less accurate at extreme body compositions.
Can RFM predict health risks?
Body fat percentage (which RFM estimates) is associated with cardiometabolic risk. Higher body fat is linked to increased risk of type 2 diabetes, cardiovascular disease, and metabolic syndrome. Additionally, because RFM incorporates waist circumference, it inherently captures some information about central adiposity, which is a particularly strong predictor of metabolic risk.
Is RFM validated for all ethnicities?
The original RFM validation used a diverse NHANES sample including multiple ethnic groups. However, like all anthropometric formulas, RFM may have different accuracy across populations. The formula was primarily derived from and validated in US adults, and results should be interpreted with appropriate clinical context for all populations.