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Relative fat mass (RFM) calculator
Short answer
Relative fat mass estimates body fat percentage from height and waist: RFM = 64 − 20 × (height ÷ waist), plus 12 for women. At 170 cm tall with an 85 cm waist, that is 36.0% for a woman and 24.0% for a man. It was built on US adults aged 20 and over against DXA scans.
Last reviewed 28 October 20261 published sourcesNot medical advice
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Get Reps freeRelative fat mass (RFM) calculator
For adults 20 and over. Keep the tape level around your waist and measure after breathing out normally. No scale needed.
Enter your waist and height.
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| Waist | Women | Men |
|---|---|---|
| 70 cm (28 in) | 27.4 | 15.4 |
| 80 cm (31 in) | 33.5 | 21.5 |
| 90 cm (35 in) | 38.2 | 26.2 |
| 100 cm (39 in) | 42.0 | 30.0 |
| 110 cm (43 in) | 45.1 | 33.1 |
Where the equation comes from
Woolcott and Bergman explored more than 350 anthropometric indices for a simple equation that matched to DXA body fat in US adults aged 20 and over from NHANES 1999–2004, then validated the winning equation in NHANES 2005–2006 (Woolcott & Bergman, 2018).
To compare RFM with BMI, they defined body-fat obesity as DXA body fat of 33.9% or more for women and 22.8% or more for men, the cut-points between the lowest and second fifths of each sex. Against that line, total misclassification in the validation data was 12.7% with RFM vs 56.5% with BMI in women, and 9.4% vs 13.0% in men (Woolcott & Bergman, 2018).
Other body fat estimates
Try the hip-based body adiposity index calculator or the body fat calculator, which uses the US Navy tape method.
Body changes, tracked over time
Reps keeps your weight and body measurements from Apple Health next to your nutrition and training, so a trend is easy to see.
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Common questions
How is relative fat mass calculated?
RFM = 64 − 20 × (height ÷ waist) for men and 76 − 20 × (height ÷ waist) for women, with height and waist in the same unit. The answer is an estimate of body fat percentage.
What is a high RFM?
The study did not set health categories. To compare RFM with BMI, it used DXA body fat of 33.9% for women and 22.8% for men, the line between the lowest fifth of US adults and the rest, and called that choice arbitrary.
Is RFM more accurate than BMI?
In the US validation data, RFM labelled body-fat obesity with fewer errors than BMI, especially in women. It is still an estimate from two measurements, not a body scan.
Sources
Figures on this page were checked against these publications on 28 October 2026.
- 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. Scientific Reports. 2018;8(1):10980. doi.org/10.1038/s41598-018-29362-1
More body tools
- BMI calculatorBody mass index with standard and lower thresholds, and when not to trust it.
- Waist-to-height ratioCheck central fat with the 'keep your waist under half your height' rule.
- Body fat calculatorBody fat percentage from a tape measure with the US Navy equations.
- Lean body mass calculatorLean mass from height and weight with three equations, or from your body fat.