Body composition calculator
RFM calculator
Relative Fat Mass — body-fat percentage from height and waist alone, validated against DXA scans. Free, instant, no sign-up.
Waist is the most movable number on this page — the coach builds the plan to shift it.
What is Relative Fat Mass?
RFM is a body-fat estimate published by Woolcott and Bergman in Scientific Reports in 2018. They tested hundreds of anthropometric combinations against DXA-measured body fat in a large NHANES sample and found that height, waist, and sex alone gave the lowest error of any simple formula. In their validation set it predicted whole-body fat percentage more accurately than BMI, with fewer missed cases of body-fat-defined obesity.
How this calculator works
Two constants, one ratio. For men: RFM = 64 − 20 × (height ÷ waist). For women: RFM = 76 − 20 × (height ÷ waist). Height and waist just have to share a unit — the ratio cancels it out, so centimetres and inches give the same answer. Measure the waist at the navel, relaxed, at the end of a normal breath out.
RFM vs the Navy method and BMI
BMI cannot tell muscle from fat, which is why a lifter and a sedentary person at the same weight score the same. RFM fixes that by reading waist size rather than total mass. It also asks for less than the US Navy tape method, which needs a neck measurement (and hips for women) on top of height — run our body fat calculator for that one and compare the two. Different equations, same job: a cheap estimate you can repeat weekly.
How to use the number
The category bands are the standard body-fat ranges, not part of the RFM equation itself — they are a reading of the number, and the boundaries are softer than they look. As with any tape-measure estimate, the trend across a couple of months is the signal; a single reading is not. This is an estimate for training decisions, not a medical assessment.