Body mass index is easy to calculate, but it says little about how much of your weight is fat. Researchers have therefore looked for simple formulas that use a tape measure and come closer to a true body fat percentage. One of the most recent is relative fat mass, or RFM, introduced in 2018 by Orison Woolcott and Richard Bergman. It needs only your height and your waist circumference, plus your sex, and was designed to estimate whole-body fat measured by DXA more accurately than BMI.
Like every formula in this class, it estimates; it does not measure. This calculator also shows the waist-to-height ratio and, if you enter your weight, the fat and lean mass implied.
How to use the relative fat mass calculator
- Select your sex, because the equation adds a constant for women.
- Enter your height in centimetres, inches, feet or metres.
- Measure your waist around the abdomen at the navel, relaxed, after breathing out, and enter it.
- Enter your weight if you want the fat and lean mass in kilograms or pounds.
- Read the RFM and its category.
Relative fat mass formula
Fat mass = weight × RFM ÷ 100
Lean mass = weight − fat mass
Height and waist must be in the same unit. The equation is from Woolcott OO and Bergman RN, Relative fat mass (RFM) as a new estimator of whole-body fat percentage: a cross-sectional study in American adult individuals, Scientific Reports 2018;8:10980. The authors derived it from DXA scans in the US National Health and Nutrition Examination Survey (NHANES) and validated it in a separate sample.
Worked example
A man 178 cm tall with an 86 cm waist, weighing 80 kg.
- Height ÷ waist = 178 ÷ 86 = 2.070
- RFM = 64 − 20 × 2.070 + 0 = 22.6% (average category)
- Fat mass = 80 × 22.6% = 18.1 kg; lean mass = 61.9 kg
- Waist-to-height ratio = 86 ÷ 178 = 0.48
A woman 165 cm tall with a 76 cm waist gets 64 − 20 × 2.171 + 12 = 32.6%.
RFM for sample heights and waists
| Height | Waist | RFM, man | RFM, woman |
|---|---|---|---|
| 160 cm | 70 cm | 18.3% | 30.3% |
| 160 cm | 80 cm | 24.0% | 36.0% |
| 160 cm | 90 cm | 28.4% | 40.4% |
| 160 cm | 100 cm | 32.0% | 44.0% |
| 170 cm | 70 cm | 15.4% | 27.4% |
| 170 cm | 80 cm | 21.5% | 33.5% |
| 170 cm | 90 cm | 26.2% | 38.2% |
| 170 cm | 100 cm | 30.0% | 42.0% |
| 180 cm | 70 cm | 12.6% | 24.6% |
| 180 cm | 80 cm | 19.0% | 31.0% |
| 180 cm | 90 cm | 24.0% | 36.0% |
| 180 cm | 100 cm | 28.0% | 40.0% |
Body fat categories
| Category (ACE) | Men | Women |
|---|---|---|
| Essential fat | 2–5% | 10–13% |
| Athletes | 6–13% | 14–20% |
| Fitness | 14–17% | 21–24% |
| Average | 18–24% | 25–31% |
| Obese | 25% and above | 32% and above |
The categories are those of the American Council on Exercise, which describe typical ranges of body fat in adults; they are not clinical thresholds.
Strengths and weaknesses
- Simple. One tape measurement and no weight are needed for the percentage itself.
- Tracks central fat. Because waist enters the equation, it responds to abdominal fat, which carries more health risk than fat elsewhere.
- Not equally good for everyone. Evidence on different ethnic groups and age bands is mixed. Muscular builds can be overestimated.
- Waist-to-height ratio is linked. RFM is essentially a rescaled waist-to-height ratio; see the Waist-to-Height Ratio Calculator.
Other ways to estimate body fat
For a second opinion try the Navy tape method in the Navy Body Fat Calculator or the broader Body Fat Calculator, and look at the Body Roundness Index Calculator for a shape-based index. The Lean Body Mass Calculator estimates lean mass from height and weight, and the BMI Calculator shows the standard category. Use one method consistently to follow your own progress.
This page is for information only and is not medical advice. Speak to a doctor about your own health and any treatment decisions.
Sources
- Woolcott OO, Bergman RN. Relative fat mass (RFM) as a new estimator of whole-body fat percentage — a cross-sectional study in American adult individuals. Sci Rep 2018;8:10980.
- American Council on Exercise. What Are the Guidelines for Percentage of Body Fat Loss? (body fat categories).
Frequently asked questions
What is relative fat mass?
Relative fat mass (RFM) is a simple equation proposed in 2018 to estimate whole-body fat percentage from height and waist circumference alone, with a correction for sex. It was designed as an easy alternative to BMI that tracks body fat more closely.
How accurate is RFM?
In the original study (Woolcott and Bergman, Scientific Reports 2018;8:10980) RFM predicted DXA-measured body fat better than BMI in a US adult sample, and the result was supported by other populations, but errors of several percentage points remain for individuals. Later studies report mixed results across ethnic groups, so treat it as an estimate.
Why does the formula need only two measurements?
Waist relative to height is a good proxy for central fat, which is a large part of total body fat. Taking height ÷ waist captures it in one ratio. The sex term of 12 adds the typical difference in fat between women and men at the same ratio.
Does it work for athletes or older adults?
It was developed in adults aged 20 to 80 from a general population sample. Muscular athletes with a large waist may be overestimated, and older people who have lost height may be too. A clinician can advise on a more accurate test.
Which waist should I measure?
The same level as for waist-to-height ratio: around the abdomen at the navel, relaxed, after breathing out. The original paper used the NHANES protocol, just above the hip bone at the midaxillary line. Using a consistent site matters most.