Relative Fat Mass (RFM) Calculator

Relative fat mass (RFM) is a simple formula that turns your height, your waist size and your sex into an estimate of your body fat percentage. It needs no scales and no special equipment, only a tape measure.

What Relative Fat Mass Measures

RFM estimates the share of your body weight that is fat, as a percentage. It was built by comparing tape measurements with DXA scans (a low-dose X-ray scan that measures fat directly) in thousands of US adults¹. It does not measure where your fat sits, and it is an estimate, not a scan.

The Formula

RFM = 64 - (20 x height / waist) + (12 x sex)

Sex = 0 for men and 1 for women. Height and waist must be in the same unit.

Ages 8 to 14 (RFMp): 74 - (22 x height / waist) + (5 x sex)

Researchers Orison Woolcott and Richard Bergman published the adult formula in 2018¹ and the children's version (RFMp) in 2019³.

How This Calculator Computes Your Result

Choose imperial or metric, then enter your sex, age, height and waist. The unit cancels out, so inches and centimeters give the same answer. Adults and teens aged 15 to 19 use the adult formula, which the authors say may be used from age 15³. Children aged 8 to 14 use RFMp. Your result shows to 1 decimal place, along with your height-to-waist ratio. For adults, the tool also shows the waist size at which you would reach the cut-off for your height. The tool gives no result if your waist is one third of your height or less, because the authors advise against using RFM there³.

What Your Result Means

For adults aged 20 and over, the result is compared with the cut-offs that Woolcott and Bergman proposed in 2020²:

These cut-offs come from a study of 31,008 US adults followed for a median of 8.3 years². They were the RFM levels that best separated people who died during follow-up from those who did not, rounded to 40% and 30%². Women with an RFM of 40 to 44.9% had a 41% higher risk of death than women under 35%, and men at 30 to 34.9% had a 57% higher risk than men under 25%, after allowing for age, BMI group, smoking and other factors².

You may see lower numbers elsewhere. The original 2018 study tested RFM using DXA cut-points of 33.9% for women and 22.8% for men, but the authors said these were arbitrary and that the study was not designed to set RFM cut-offs¹.

For children and teens there are no agreed RFM cut-offs, so the tool shows the estimate only³. For under-20s, see our Kids BMI Calculator.

How to Measure Correctly

RFM was built with the waist measured at the top of the hip bone (the iliac crest), the method used in the US NHANES survey¹.

• Stand up straight with your stomach relaxed. Measure on bare skin.

• Find the top of your right hip bone by pressing your fingers along the side of your hip. Go up to the highest point of the bone, in line with your armpit⁴.

• Wrap the tape around your waist at that level, level with the floor all the way round. It should be snug but not pressing into the skin⁴.

• Read the tape at the end of a normal breath out⁴.

• Measure twice and use the average.

The World Health Organization method measures halfway between your lowest rib and the top of your hip bone⁵. Readings at that midpoint tend to be several centimeters smaller than readings over the hip bone⁸, which would make your RFM read low. Small differences add up: for a man who is 5 ft 10 in, each extra inch of waist raises RFM by about 1 point.

RFM vs BMI: What the Evidence Shows

BMI uses weight and height, so it cannot tell fat from muscle. In the 2018 study, RFM matched DXA body fat more closely than BMI did. It explained 69% of the variation in women (BMI 65%) and 75% in men (BMI 61%)¹. A BMI of 30 missed 72% of women whose DXA scan showed obesity-level fat, while RFM missed 5%¹. The trade-off was more false alarms: RFM flagged 41% of women whose scan was below the cut-point, against none for BMI¹.

Later studies link RFM to health outcomes. In 8,295 Dutch adults followed for 11 years, each standard step up in RFM raised heart failure risk by 67%, against 28% for BMI⁶. In 46,535 US adults, RFM predicted death from diabetes-related causes better than BMI or waist size alone⁷. For death from any cause or from heart disease, RFM, BMI and waist size were about equally strong⁷.

How RFM Differs From the Navy Method

Our Body Fat Calculator uses the US Navy tape method, which needs your neck measurement as well (and your hips if you are a woman). RFM needs only your height and waist. It was also built against DXA scans of the general US population¹. The two methods can give different numbers for the same person. Treat each as an estimate, and use the same method each time if you are tracking change.