RFM Calculator (Relative Fat Mass from height and waist)

Understanding the relative fat mass (RFM) calculator — concept and principle

Relative fat mass (RFM) estimates your body fat percentage from just height and waist circumference. It was proposed by Woolcott and Bergman (2018) as a simple alternative to BMI that better reflects body fat. You only need a tape measure.

Height and waist: with the default inputs the result is 25.1% (at or above 22.8% cut-off).

How it is calculated

Men: RFM = 64 − 20 × (height ÷ waist)

Women: RFM = 76 − 20 × (height ÷ waist)

Height and waist must be in the same unit (cm here).
Measure the waist at the top of the hip bone, as in the NHANES survey.
The calculator compares the result with the DXA body fat cut-offs the authors used in their analysis: 22.8% for men and 33.9% for women.

The principle

Waist size reflects abdominal fat, and height scales the body. A larger waist relative to height means more body fat. The authors tested many combinations of simple measurements in US adults (NHANES 1999–2004) against body fat measured by DXA, found that the inverse of the waist-to-height ratio predicted it well, and then checked the formula in a separate NHANES sample. Women have more body fat than men at the same proportions, which is why the constant is 12 points higher.

Worked example

Default inputs: male, height 175 cm, waist 90 cm.

Height ÷ waist = 175 ÷ 90 = 1.94.
RFM = 64 − 20 × 1.944 = 64 − 38.89 = 25.1%.
Study cut-off for men = 22.8% → At or above.

How to read the result

The result is an estimate of body fat percentage. The authors' cut-offs were chosen for their analysis, not as official obesity thresholds. RFM is most useful for tracking change in the same person or as a quick screening alongside BMI and waist size. It is less accurate in very muscular people and has been validated mainly in US adults.

Common mistakes

Measuring the waist at the navel instead of the top of the hip bone, or over clothing.
Mixing inches and centimetres.
Treating the study cut-off as a diagnosis of obesity.

Keep in mind: This is an estimate, not a diagnosis. Discuss weight and health concerns with a doctor.

Frequently asked questions

Is RFM better than BMI?

In the original study it estimated DXA body fat more accurately than BMI, but both are simple screening tools.

Can RFM be used in children?

No. It was developed for adults aged 20 and over.

How do I measure my waist?

Stand relaxed, find the top of your hip bone and wrap the tape horizontally at that level after breathing out.

Sources: 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", Sci Rep 2018;8:10980

Checked 9 Nov 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;8:10980 (equation, sex coding, NHANES samples, DXA cut-points of 22.8% and 33.9%).

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