Relative Fat Mass (RFM) Calculator

Relative Fat Mass (RFM) is an equation proposed by Woolcott and Bergman in 2018 to estimate whole-body fat percentage from height and waist circumference. It uses a sex coefficient of 0 for men and 1 for women, matching the groups in the original study.

The equation was developed with 12,581 US NHANES participants aged 20–85 (1999–2004 data) and validated with 3,456 aged 20–69 (2005–2006 data). The study compared estimates against DXA body composition measurements.

BMI relates weight to height. RFM uses height and waist circumference to estimate a body fat percentage; weight is not part of this equation. In the original validation sample, RFM estimated DXA-measured whole-body fat percentage more accurately than BMI overall. That population result does not establish accuracy for every individual or group.

For a weight-based index, see the BMI worksheet.

Measure standing height and waist circumference in the same units: centimeters for metric, or inches for imperial. The study measured the unclothed waist with a horizontal tape at the uppermost lateral border of the right ilium (hip bone), at the end of an exhalation. Measuring at the navel can produce a different result.

Enter ordinary positive decimal numbers using a period or comma as the decimal separator. Switching units clears the entries so the previous numbers are not reinterpreted in a different unit.

RFM (%) = 64 − 20 × (height ÷ waist) + 12 × sex
sex = 0 for male; sex = 1 for female

Both lengths must have the same unit. The published equation states meters for both; using centimeters for both or inches for both gives the same ratio. For a man 180 cm tall with a 90 cm waist: 64 − 20 × (180 ÷ 90) = 24%. For a woman with the same measurements, add 12 to get 36%.

The equation can produce a negative number for some positive measurements. A negative body fat percentage has no physical meaning, so the worksheet reports no estimate if the calculation lies outside 0–100%.

The result is a prediction of whole-body fat percentage, not a direct measurement, diagnosis, or clinical category. The original study did not propose RFM cutoffs for diagnosing obesity.

Validation used a US adult sample, with subgroup analyses for Mexican-American, European-American, and African-American participants. The authors called for research in other ethnicities, children, athletes, and people with specific diseases. The validation sample extended only to age 69; the form does not collect age and cannot tell whether a user matches the study population.

Why is sex used in the formula?

The published equation includes a fixed 12-point coefficient for the female group in its data. Select the coefficient matching the group used for the equation; this binary model does not represent every person's body composition.

Can I enter inches?

Yes. Height and waist are divided, so the units cancel if both measurements use inches.

Source: Woolcott OO and Bergman RN, Scientific Reports, published July 20, 2018. Source checked September 29, 2026.

Related worksheets: ABSI and waist-to-height ratio.