About the Relative Fat Mass Calculator
Relative fat mass (RFM) is a simple equation, published by Woolcott and Bergman in 2018, that estimates whole-body fat percentage from just two tape-measure readings: height and waist circumference. It was developed and validated against DXA body-composition scans in more than 12,000 US adults and was found to estimate body fat more accurately than BMI, particularly for women.
Enter your sex, height and waist; add your weight if you also want fat mass and lean mass in pounds or kilograms. The result is placed in the familiar American Council on Exercise (ACE) body-fat categories so you can see whether you are in the athletic, fitness, average or obese range.
Because RFM relies on the waist-to-height relationship, it is quick and cheap to track over time — measure your waist the same way each time. It was developed in adults aged 20 and over and is not intended for children, pregnancy or people with abdominal conditions that change waist size. It is an estimate, not a medical measurement.
With the default inputs, the body fat (rfm) is 22.8%. Change any value above to recalculate instantly.
How to use the relative fat mass calculator
- 1Choose US or metric units and select your sex.
- 2Enter your height.
- 3Measure your waist just above the top of your hip bone after breathing out, and enter it.
- 4Optionally enter your weight to see fat and lean mass.
- 5Read your estimated body fat and category.
Formula and method
Height and waist must be in the same unit, so the ratio has no unit. Taller people with a smaller waist have a larger height-to-waist ratio and therefore a lower estimated body fat; women receive a constant 12-point increase because they carry more essential fat at the same proportions.
Fat mass is body weight × RFM ÷ 100 and lean mass is the remainder. Categories use the ACE body-fat chart: for men 6–13% athletes, 14–17% fitness, 18–24% average and 25%+ obese; for women 14–20%, 21–24%, 25–31% and 32%+. In the original NHANES-based study, waist was measured just above the top of the hip bone (iliac crest).
- RFM
- Relative fat mass — estimated body fat %
- height
- Standing height (same unit as waist)
- waist
- Waist circumference
- S
- Sex: 0 = male, 1 = female
Worked examples
Man, 5 ft 10 in, 34-inch waist, 170 lb
Height is 70 inches, so 64 − 20 × (70 ÷ 34) = 64 − 41.18 = 22.8% body fat — the average range for men. Of 170 lb, about 38.8 lb is fat and 131.2 lb lean mass.
Woman, 165 cm, 80 cm waist, 65 kg
64 − 20 × (165 ÷ 80) + 12 = 64 − 41.25 + 12 = 34.75%. That is in the ACE obese range for women, equal to about 22.6 kg of fat mass at 65 kg.
Man, 180 cm, 100 cm waist, 95 kg
64 − 20 × 1.8 = 28% body fat, above the 25% ACE threshold for men, or about 26.6 kg of fat. His waist-to-height ratio of 0.56 also signals raised central fat.
Frequently asked questions
What is relative fat mass (RFM)?+
RFM is a formula that estimates body fat percentage from height and waist circumference. It was published in Scientific Reports in 2018 by Woolcott and Bergman and validated against DXA scans.
Is RFM more accurate than BMI?+
In the original research RFM predicted DXA-measured body fat better than BMI and classified obesity more accurately, especially in women, because it reflects where fat is stored rather than total weight.
Where should I measure my waist for RFM?+
The study used NHANES measurements, taken just above the uppermost edge of the hip bone (iliac crest). Stand relaxed, breathe out normally and keep the tape snug but not compressing the skin. Measure the same way each time.
What is a healthy body fat percentage?+
Using ACE categories, about 14–24% is typical for fit and average men and 21–31% for women. Essential fat is roughly 2–5% for men and 10–13% for women; very low levels are unhealthy.
Can I use RFM to track progress?+
Yes. Because it needs only a tape measure, RFM is easy to repeat monthly. A shrinking waist at the same height lowers your RFM, even if the scale weight changes little.
This tool gives general estimates and is not medical advice. Talk to a doctor or qualified professional about your health, diet or training.