How the RFM Calculator works
The Relative Fat Mass (RFM) calculator estimates whole-body fat percentage from three simple inputs: sex, height, and waist circumference. It does not require a scale that measures body composition, skinfold calipers, or a bioelectrical impedance device — only a tape measure and your height.
The Woolcott-Bergman formula
- Men: RFM = 64 − (20 × height ÷ waist circumference)
- Women: RFM = 76 − (20 × height ÷ waist circumference)
Height and waist circumference must be entered in the same unit (this calculator uses centimeters for both), because the formula only uses their ratio. The result is an estimated percentage of body weight that is fat mass. Multiplying that percentage by body weight gives an estimated fat mass in kilograms; subtracting fat mass from total weight gives an estimate of fat-free (lean) mass.
Where the formula comes from
Woolcott and Bergman published the RFM equation in 2018 as a simpler, waist-based alternative to BMI for estimating body fat percentage in adults. It was derived and validated against dual-energy X-ray absorptiometry (DXA) measurements in a large U.S. adult sample, and in that validation it tracked body fat percentage more closely than BMI, particularly across different body shapes.
Measuring waist circumference accurately
- Measure at the end of a normal exhale, standing, with the tape snug but not compressing the skin.
- Use the midpoint between the top of the hip bone and the bottom of the lowest rib, or measure at the navel if that landmark is easier to find consistently — pick one method and stay consistent for follow-up comparisons.
- Take the average of two or three measurements if readings vary by more than a centimeter or two.
When to consult a professional
This tool performs the standard RFM arithmetic for estimation and tracking. It is not a diagnosis and does not replace a clinical body-composition assessment. For health decisions, consult a healthcare provider.