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What is RFM and why is it strong?
RFM (Relative Fat Mass) is a simple yet powerful formula that estimates body fat percentage from the height and waist circumference ratio alone (Woolcott & Bergman, 2018; Cedars-Sinai). Because it was developed against DXA (the gold-standard body scan) in thousands of people, it estimates fat percentage more accurately in most people than other circumference-based methods. Unlike BMI, it separates muscle from fat, and it requires fewer measurements than the US Navy method (no neck or hip needed).
How is RFM calculated?
The formula uses only the height-to-waist ratio and sex. Height and waist are entered in the same unit:
Female: RFM = 76 − 20 × (Height ÷ Waist)
Height and waist in the same unit (cm); the result is fat percentage
Example: Male, height 178 cm, waist 90 cm:
64 − 20 × (178 ÷ 90) = 64 − 39.6 = 24.4% estimated fat percentage. In the ACE classification, this falls in the "average" (18–24%) range for men.
Method: Woolcott OO & Bergman RN, Scientific Reports, 2018 (developed against DXA). Category bands: American Council on Exercise (ACE) — men athletic 6–13% / fitness 14–17% / average 18–24% / obese ≥25%; women athletic 14–20% / fitness 21–24% / average 25–31% / obese ≥32%.
Ways to use RFM correctly
- Measure your waist accurately: RFM relies entirely on the height-to-waist ratio; a measurement error of a few centimeters noticeably changes the result. Always measure at the same point (navel level), on an empty stomach.
- Trend > single value: The change over time matters more than the absolute percentage. Measure weekly under the same conditions.
- Compare with US Navy: Using the two methods side by side (there is a body fat percentage tool on this site) makes the estimate more reliable.
- DXA for precision: If you need athletic or clinical precision, measurement methods such as DXA/BIA are required; RFM is an estimate.
Frequently Asked Questions
Because RFM was developed against DXA, it is among the more accurate circumference-based methods; even so, it is an estimate and can be off by a few points from person to person. For an exact value, DXA or bioelectrical impedance (BIA) is needed.
RFM requires fewer measurements (only height + waist; no neck or hip needed) and was developed against DXA. The US Navy method is also widely used and reliable. Using both together and comparing them is the most robust approach.
According to ACE, the "fitness" range is 14–17% for men and 21–24% for women; "average/acceptable" is 18–24% for men and 25–31% for women. The essential fat women need for reproductive health is higher than in men.
The essential fat women need for hormonal and reproductive function (about 10–13%) is higher than in men (2–5%). That is why, even at the same fitness level, a healthy fat percentage for women is a few points above that of men.
For the fat percentage, no — height and waist alone are enough. If you enter your weight, the tool also calculates your fat mass (kg) and lean mass (kg); this is useful for tracking muscle/fat changes.
Since RFM depends on the waist-to-height ratio, reducing your waist circumference (calorie balance, a protein- and fiber-focused diet, strength + cardio, sleep) lowers fat percentage. Preserving muscle mass ensures the loss comes from fat.