Relative fat mass calculator (RFM)

The formula

The authors tested 365 combinations of body measurements against body fat measured by DXA scan and picked the simplest ones that tracked it best (Woolcott 2018). The final equations use only height and waist (Woolcott 2018):

Women: RFM = 76 − (20 × height ÷ waist)
Men: RFM = 64 − (20 × height ÷ waist)

Or as one equation, RFM = 64 − (20 × height ÷ waist) + (12 × sex), where sex is 0 for men and 1 for women (Woolcott 2018). The authors used meters and rounded the numbers for practical use (Woolcott 2018). Because height and waist are divided, any unit works as long as both are the same.

The result is an estimate of whole-body fat percentage (Woolcott 2018). For example, a man 175 cm tall with a 100 cm waist has RFM = 64 − (20 × 175 ÷ 100) = 64 − 35 = 29.0%. A woman 162 cm tall with an 85 cm waist has RFM = 76 − (20 × 1.906) = about 37.9%. The paper prints no worked example; these are worked from the formula.

Adding age or ethnicity to the formula did not make it fit body fat better, so the authors left them out (Woolcott 2018).

How well it matched DXA scans

In the validation group (NHANES 2005 to 2006), RFM tracked DXA body fat more closely than BMI did (Woolcott 2018):

From the Results and Table 2 of the paper (Woolcott 2018). R² shows how much of the spread in body fat a measure explains (1 is perfect). RFM also performed better than BMI across age groups (Woolcott 2018).

Is there a cutoff?

No. The authors wrote that their study was not designed to propose RFM cutoffs for obesity (Woolcott 2018). There is also no agreed definition of obesity by body fat percentage (Woolcott 2018). To compare RFM with BMI, they picked an arbitrary line: DXA body fat of 33.9% or more in women and 22.8% or more in men (Woolcott 2018). Using that line, RFM missed fewer people with high body fat than BMI (5.0% vs 72.0% of such women, 3.8% vs 4.1% of such men) (Woolcott 2018). But in women, RFM also flagged more people whose DXA body fat was below the line (41.0% vs 0% with BMI); in men it flagged fewer (32.3% vs 49.7%) (Woolcott 2018). That line is a study device, so the calculator shows your estimate without a label.

Limits

DXA, the reference the formula was built on, tends to read low at lower body fat and in men, and high at higher body fat and in women, so RFM carries those errors too (Woolcott 2018). Results by ethnicity cover only Mexican American, European American and African American adults; the authors say RFM still needs testing in other groups, children, athletes and people with specific diseases (Woolcott 2018). Measure your waist the way the survey did: standing, at the top of the right hip bone, at the end of a normal breath out (Woolcott 2018). A high estimate is a reason to talk to your doctor about your weight and other heart and metabolic risk factors, not a diagnosis.

Related checks

The BMI calculator uses weight and height and is what RFM was compared with. The waist-to-height ratio uses the same two measurements and has guideline bands from NICE. The body roundness index and the body shape index (ABSI) are body shape scores from waist, height and, for ABSI, weight.