Last updated October 6, 2026 · Report an error
Relative Fat Mass (RFM) is a body composition formula published by Woolcott & Bergman in 2018, designed to estimate whole-body fat percentage using nothing more than height and waist circumference — no scale, no calipers, and no specialized equipment required. It was developed specifically to address a well-known weakness of Body Mass Index (BMI): BMI uses only height and weight, so it can't distinguish between muscle mass and fat mass, which means a muscular athlete and a person carrying more body fat can end up with an identical BMI despite very different actual body compositions. RFM instead incorporates waist circumference, which correlates more directly with abdominal and visceral fat than weight alone does. In the original published research, the authors validated RFM against DEXA (dual-energy X-ray absorptiometry) scans — a gold-standard imaging method for measuring actual body fat percentage — and found that RFM tracked DEXA-measured fat percentage more closely than BMI did across the study population. This calculator is for general reference and educational purposes only — it is not a medical diagnosis. Consult a doctor or qualified professional for an actual body composition assessment.
How it works
Enter your sex, height, and waist circumference in the same unit system. The tool applies the sex-specific RFM formula published by Woolcott & Bergman: 64 minus (20 times height divided by waist) for men, or 76 minus (20 times height divided by waist) for women. The different constants (64 vs. 76) account for typical physiological differences in fat distribution between sexes, based on the original study's statistical fit against DEXA scan data.
- Enter sex.
- Enter height (cm).
- Enter waist circumference (cm).
- Click Calculate to see your results.
Examples
A man, 170cm tall with an 80cm waist
RFM = 64 - (20 x 170/80) = 64 - 42.5 = 21.5%.
A woman, 165cm tall with a 75cm waist
RFM = 76 - (20 x 165/75) = 76 - 44 = 32%, reflecting the higher baseline constant used for women in the formula.
A man with a larger waist relative to height
A man 175cm tall with a 100cm waist: RFM = 64 - (20 x 175/100) = 64 - 35 = 29%, showing how a larger waist-to-height ratio increases the estimated fat percentage.
Who should use it
- Estimating whole-body fat percentage without specialized equipment.
- Comparing against BMI and other body composition metrics.
- Tracking body composition trends over time between clinical checkups.
Industry applications
- Fitness assessment
- Public health research
Advantages
- Requires only two simple measurements.
- Validated against DEXA scans in the original published research.
- Addresses a known weakness of BMI by accounting for waist circumference.
Limitations
- Still an estimate, not a direct body composition measurement.
- Less familiar to the general public than BMI.
- May be less reliable for children, adolescents, or highly muscular individuals.
Common mistakes to avoid
- Measuring waist circumference inconsistently between checks.
- Treating RFM as a precise clinical diagnosis rather than a validated estimate.
- Mixing unit systems (like a height in centimeters with a waist measurement in inches), which produces a meaningless result.
Best practices
- Measure waist circumference at the navel, standing relaxed, for the most consistent reading.
- Use the same measurement technique and time of day each time you check, for the most meaningful comparison over time.
- Treat RFM as one data point among several (alongside a doctor's assessment, BMI, or other measures) rather than a standalone diagnosis.
Tips
- Track RFM over time using the same measurement technique for the most meaningful trend.
- Discuss any body composition estimate with a doctor or qualified professional, especially if you have health concerns tied to body fat percentage.