What relative fat mass measures
Relative fat mass (RFM) is a simple estimate of body fat percentage published in 2018 by Orison Woolcott and Richard Bergman at Cedars-Sinai Medical Center. They searched hundreds of body measurement formulas against DXA scans, the reference method for body composition, in adults from the US National Health and Nutrition Examination Survey (NHANES). The best simple formula used only height and waist:
- Men: RFM = 64 − 20 × (height ÷ waist)
- Women: RFM = 76 − 20 × (height ÷ waist)
Height and waist must be in the same unit, inches or centimeters. The result reads directly as an estimated body fat percentage.
Worked example
A man is 69 inches tall with a 34-inch waist and weighs 180 lb.
- Height ÷ waist: 69 ÷ 34 = 2.03.
- RFM: 64 − 20 × 2.03 = 23.4%, in the average range for men.
- Fat mass: 180 × 23.4% = 42.1 lb; lean mass about 137.9 lb.
- Waist-to-height ratio: 34 ÷ 69 = 0.49, just under the 0.5 threshold.
- BMI for comparison: 26.6, which BMI alone would call overweight.
Body fat ranges
These ranges from the American Council on Exercise are widely used to read a body fat percentage:
| Category | Men | Women |
|---|---|---|
| Essential fat | 2 to 5% | 10 to 13% |
| Athletes | 6 to 13% | 14 to 20% |
| Fitness | 14 to 17% | 21 to 24% |
| Average | 18 to 24% | 25 to 31% |
| Obesity | 25% and above | 32% and above |
Why the waist matters
BMI uses weight and height, so it cannot tell muscle from fat: a muscular athlete and a sedentary person of the same weight get the same BMI. The waist, on the other hand, tracks the fat stored around the abdomen, which is the type most linked to type 2 diabetes, high blood pressure and heart disease. In the original study, RFM misclassified fewer people as obese or not obese than BMI did.
How to measure
- Stand straight with your feet together and your belly relaxed.
- Place the tape horizontally just above the top of the hip bones, snug but not compressing the skin.
- Read the tape at the end of a normal breath out, and repeat twice.
Limits
RFM is an estimate. It was developed in American adults and may be less accurate in children, older adults, pregnant women and some ethnic groups, and it says nothing about where fat is stored beyond the waist. Use it to follow your own trend over time rather than as a diagnosis.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.