What Is the RFM Calculator?
This RFM calculator (relative fat mass calculator) estimates your body fat percentage from just two body measurements: your height and your waist circumference. No scale, no special equipment and no age are needed, only a tape measure.
The relative fat mass index was introduced in 2018 by Orison Woolcott and Richard Bergman. They built it on data from thousands of American adults in the NHANES survey, whose body fat had been measured with DXA scans. In that data, RFM estimated body fat percentage more accurately than the well-known body mass index (BMI), which often misclassifies obesity: many people with a "normal" BMI actually carry too much fat.
Want to know how to calculate your fat mass? Enter your measurements above, or read on to learn how the relative fat mass index works and how to interpret it.
We try our best to make this calculator as precise and reliable as possible. However, it can never replace a professional doctor's assessment. If any health condition bothers you, consult a physician.
Body Fat Mass — an Essential Part of the Human Body
Body fat, also called adipose tissue, is an essential component of the human body. Its primary function is energy storage, but it does much more: it cushions and protects internal organs, insulates the body and works as an endocrine organ that produces hormones such as leptin.
The percentage of body fat varies with age and sex. Women naturally have a higher body fat percentage than men, because of reproductive and hormonal functions, and body fat tends to rise with age.
A certain amount of fat is vital, but too much of it harms health. Research links a high body fat percentage with higher morbidity and mortality, which is why excessive accumulation of body fat is classified as a disease: obesity. Obesity is an important risk factor for many other disorders, including cardiovascular disease, type 2 diabetes and several forms of cancer. Fat stored around the waist (visceral fat) is especially linked with these risks, and that is exactly the fat a waist measurement captures.
How to Calculate Your Fat Mass?
There are many ways to measure body fat. The most sophisticated include underwater weighing (hydrostatic weighing) and whole-body air displacement plethysmography. More common ones are dual-energy X-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA).
Unfortunately, all of these techniques require specialist equipment, and they are not always available, or cheap. That is why researchers created mathematical formulas that estimate body fat from simple measurements. The RFM formula is one of the simplest of them.
The RFM Formula
Men: RFM = 64 − 20 × (height / waist circumference)
Women: RFM = 76 − 20 × (height / waist circumference)
Height and waist circumference must be in the same unit. Because the formula uses their ratio, the unit cancels out: centimeters, inches or feet and inches all give exactly the same result. The number you get is read directly as a body fat percentage. The higher starting value for women (76 instead of 64) reflects their naturally higher share of essential fat.
If you also enter your weight, the calculator turns the percentage into kilograms and pounds:
- Fat mass = weight × RFM / 100
- Lean mass = weight − fat mass (muscles, bones, organs and water)
You can estimate your body composition in other ways, too: try the body fat calculator, the Navy body fat calculator or the BAI calculator, and compare the results.
How to Measure Your Waist Correctly
RFM depends on a single tape measurement, so take it carefully:
- Stand straight with your feet together, arms relaxed, and uncover your abdomen.
- Find the top of your hip bones (the iliac crest) on both sides. This is where waist circumference was measured in the NHANES survey the formula comes from.
- Wrap the tape around your body at that level, keeping it horizontal all the way around.
- Keep the tape snug, but do not compress the skin, and read it at the end of a normal exhale. Do not pull your stomach in.
- Measure two or three times and use the average.
For a man 180 cm (5 ft 11 in) tall with an 88 cm (34.6 in) waist, a 2 cm (0.8 in) error in the waist changes RFM by about 0.9 percentage points.
How to Use the RFM Calculator
- Choose your sex: the formula is different for men and women.
- Pick the unit system: US (feet, inches, pounds) or metric (centimeters, kilograms). If you switch after typing, your values are converted automatically.
- Enter your height and waist circumference.
- Optionally enter your weight to see your fat mass and lean mass and a comparison with BMI.
- Press Calculate. You will see your RFM, its category on the relative fat mass chart, whether you reach the RFM obesity cut-off, the formula with your own numbers, and the waist measurements that correspond to each category at your height.
Relative Fat Mass Index and Relative Fat Mass Chart
RFM is a body fat percentage, so it is interpreted with a body fat chart. The calculator uses the widely used chart of the American Council on Exercise (ACE):
| Category | Women | Men |
|---|---|---|
| Essential fat | 10–13% | 2–5% |
| Athletes | 14–20% | 6–13% |
| Fitness | 21–24% | 14–17% |
| Average | 25–31% | 18–24% |
| Obese | 32% and more | 25% and more |
The calculator shows RFM with one decimal place and classifies the value it displays, so each range runs up to the next one: for women, "Athletes" covers 14.0–20.9%, "Fitness" 21.0–24.9%, and so on. Values below essential fat (under 10% for women, under 2% for men) are not healthy and almost always point to a measuring mistake.
The RFM Obesity Cut-off
Besides the general ACE chart, the authors of RFM later tested which values are linked with a higher risk of death. In a 2020 study they proposed specific cut-offs for diagnosing obesity with RFM:
- Women: RFM of 40% or more
- Men: RFM of 30% or more
These cut-offs are stricter than the ACE "obese" category. A woman with an RFM of 36% is "obese" on the ACE chart but still below the RFM cut-off. The calculator shows both and explains in words where your result stands.
RFM Calculator – a Practical Example
A man, metric units
- Height = 180 cm, waist circumference = 88 cm, weight = 82 kg
- Height / waist = 180 / 88 = 2.0455
- RFM = 64 − 20 × 2.0455 = 64 − 40.91 = 23.09 ≈ 23.1%
- Fat mass = 82 × 23.1 / 100 = 18.9 kg, lean mass = 82 − 18.9 = 63.1 kg
For men, 18–24.9% is the average category, and 23.1% is well below the RFM obesity cut-off of 30%. Interestingly, his BMI is 25.3 kg/m², which BMI calls "overweight". RFM, looking at the waist, puts his body fat in the typical range.
The same man in US units
- Height = 5 ft 11 in = 71 in, waist circumference = 35 in, weight = 180 lb
- RFM = 64 − 20 × (71 / 35) = 64 − 20 × 2.0286 = 64 − 40.572 ≈ 23.4% (average)
- Fat mass = 180 × 23.4 / 100 = 42.1 lb, lean mass = 137.9 lb
The result differs slightly from the metric example only because 5 ft 11 in and 35 in are not exactly 180 cm and 88 cm.
Two women, 165 cm (5 ft 5 in) tall
- Waist 72 cm (28.3 in): RFM = 76 − 20 × (165 / 72) = 76 − 20 × 2.2917 = 76 − 45.834 ≈ 30.2%, the average category for women (25–31.9%).
- Waist 84 cm (33.1 in): RFM = 76 − 20 × (165 / 84) = 76 − 20 × 1.9643 = 76 − 39.286 ≈ 36.7%. That is obese on the ACE chart (32% and more) but 3.3 percentage points below the RFM obesity cut-off of 40%. At this height, the cut-off is reached with a waist of 91.6 cm (36.1 in).
RFM vs BMI
BMI divides weight by height squared. It is easy to compute, but it cannot tell fat from muscle and ignores where fat is stored. As a result, a muscular person can be labeled "obese", while a person with a normal weight but a large waist may be told everything is fine. RFM avoids weight altogether and uses the waist, which reflects abdominal fat. In the original study it identified people with excess body fat more reliably than BMI, in both women and men.
If you enter your weight, the calculator shows your BMI next to RFM and tells you when the two disagree about obesity.
Limitations of RFM
- It is an estimate. For an individual, RFM can differ from a DXA measurement by several percentage points.
- It was developed for adults aged 20 and over. It is not meant for children, teenagers or pregnant women.
- Unusual body builds, such as bodybuilders or people with a very wide or very narrow frame, can get a less accurate result.
- The measurement matters. Measuring at the navel instead of the top of the hip bones, or pulling the stomach in, changes the result.
Use RFM as a quick screening tool and combine it with other measurements and an overall health assessment by a medical professional.
Frequently Asked Questions
What does RFM stand for?
RFM stands for relative fat mass. It is an estimate of the percentage of your body weight that is fat, calculated from height and waist circumference.
What is a healthy RFM?
On the ACE chart, the healthy ranges are roughly 14–31.9% for women and 6–24.9% for men (from "athletes" to "average"). The RFM obesity cut-off is 40% for women and 30% for men.
Does RFM need my weight or age?
No. RFM uses only height, waist circumference and sex. Weight is optional in this calculator: it is used only to convert the percentage into kilograms and pounds of fat and to compare the result with BMI.
Why is RFM higher for women with the same measurements?
Women naturally carry more essential fat, needed for hormonal and reproductive functions. The formula reflects this with a starting value of 76 instead of 64, so at the same height and waist a woman's RFM is 12 percentage points higher than a man's.
Why do I get a negative or very low RFM?
This almost always means a unit mix-up, for example a waist entered in inches while centimeters are selected, or a measurement taken in the wrong place. Body fat cannot be negative, so the calculator reports an error instead of a number in that case.
Medical disclaimer
All information on this page is for informational purposes only and is not a substitute for medical consultation. RFM estimates body fat; it does not diagnose any disease. Always discuss your results with a health care provider.